Two Stroke Top End Rebuilding – TSM Tech
Top-end rebuilding is the most frequent and costly service routine on two-stroke dirt bikes. Every year, dirt bike riders waste loads of money on top-end parts that didn’t need to be replaced, or make costly mistakes while performing repairs. This section will give you the dos and don’ts to easy top-end rebuilding, plus some tips that aren’t printed in your factory service manual.
Before You Start
Thoroughly wash your bike because dirt stuck to the underside of the top frame tube could break loose when servicing and fall into the engine! Use a stiff plastic brush and hot soapy water to clean off the grit and grime around the base of the cylinder, on the carburetor and intake boot, and especially underneath the top frame rail. Degreaser can be used on metal surfaces, but take care not to leave it on rubber or gasket surfaces.
Tools
You’ll need at least some 3/8-inch-drive metric sockets and box wrenches (open-end wrenches can round off the edges on the cylinder or head nuts, and shouldn’t be used for top-end rebuilding), a needle-nose pliers for removing circlips, and a gasket tool to scrape the old gaskets away. For soft tools, get some shop towels, aerosol oven cleaner, a Scotch-Brite pad, a locking agent such as Loctite, a gasket scraper, a brush, and a bucket of soapy water. Regarding measuring tools, you’ll need a compression tester, a feeler gauge, and a digital vernier caliper.
Compression Testing
A compression tester is a useful diagnostic tool, and readily available from Sears or auto parts stores. Buy the threaded type and make sure the kit comes with an adapter that matches the spark plug threads of your engine. Performing a compression test is simple. Start by removing the spark plug, thread in the adapter, and hold the throttle wide open and the kill button on. This will prevent any spark and enable the engine to draw in maximum airflow. Then kick-start the engine several times until the needle on the pressure gauge peaks. The pressure reading depends on two main factors; the compression ratio and the altitude at which the engine is being tested. The compression ratio will also depend on if the engine is equipped with exhaust valves and their condition. When the exhaust valves are in the closed position the compression ratio will be greater than if the valves are carbon-seized in the open position. The difference may yield a pressure reading 25 psi. The quality of compression testers varies greatly. The main thing that a compression tester can identify is a change in condition. Whenever you rebuild the top end, take a compression pressure reading and mark it down. When the pressure changes 20% check the condition of the piston and rings. Pistons usually last twice as long as rings.
Crankcase Pressure Testing
The crankcase of a two-stroke engine is sealed from the tranny. It’s important that the two crankshaft seals be in optimum condition. One side of the crankshaft uses a dry seal and the other a wet seal. The dry seal runs on the magneto side and the wet seal runs in oil on the tranny side. When the dry seal wears, the crankcase sucks in hot air, causing the mixture to run lean and overheat the engine. When the wet seal wears, the crankcase sucks in tranny oil, causing the engine run rich and eventually wet-foul the spark plug.
A crankcase pressure test involves the use of a vacuum pump with spark plug adapter, and rubber plugs to block off the intake and exhaust manifolds of the cylinder. The piston must be positioned at BDC to allow the transfer ports to be wide open linking the bore and the crankcase. The hand-pump produces vacuum pressure up to a standard setting of 5 psi. The normal bleed-down pressure loss is 1 psi per minute. Cylinders with complicated exhaust valve systems can be difficult to block-off air leaks, and harder to test. Crankcase pressure testing kits are available from Motion Pro.
If I suspect that an engine has an air leak in the crankcases, I do a visual test. Start by power washing the engine clean. Then remove the magneto cover. Spray the magneto clean with an aerosol can of brake cleaner. Make sure to use a non-chlorinated type of cleaner (green colored can). Now spray baby powder to all the suspect areas of the engine. Spray the powder on the crankcase around the magneto, at the crankcase seam line, the cylinder base, and the reed valve. Run the engine for a while, the white baby powder will highlight any fluid or air leaks on the engine. The baby powder test is much better than the alternative test of blowing raw propane gas at different areas of a running engine and listening for a change in the idle rpm. That is dangerous because it involves flammable gas and a hot engine with random electrical shorts.
Maintenance and Inspection
A thorough top-end rebuild requires removing the reed valve, cylinder head, and cylinder. You should tear down your top end periodically and inspect the reed valve, cylinder head, cylinder, piston, and so on. Use the following chart to determine when you should tear down your bike:
Displacement: 80cc 125cc 250cc 500cc
Tear down after: 20 hours 30 hours 40 hours 60 hours
Note that air-cooled bikes should be inspected more frequently. Also, you may want to inspect more often if you are riding in fine sand or lots of mud. When you tear down the engine, inspect each system and look for the following trouble signs.
Reed Valve
Check the reed petals for open gaps between the sealing surfaces. In time, the reed petals lose their spring tension, and the back-flow can cause a flat-spot in the throttle response. Stock nylon reeds tend to split at the edges on bikes that are constantly over-revved. Expert riders find that carbon fiber reeds last much longer.
Cylinder Head
Check the head at the edge of the chamber for erosion marks—a sign that the head gasket was leaking. If the head or top edge of the cylinder is eroded, it must be turned on a lathe to be resurfaced.
Cylinder
All cylinder bases use aligning (dowel. pins around two of the cylinder base studs. These pins are made of steel, and after heavy power washing, they get corroded. That makes it difficult to remove the cylinder from the crankcases. Never use a pry bar! That will damage the cylinder. Instead use a plastic mallet to hit upward on the sides of the cylinder at a 45-degree angle. Alternate from left to right sides so the cylinder lifts up evenly. After you remove the cylinder, stuff a shop towel into the open crankcases to prevent debris from entering the engine.
The Different Types of Steel-Lined and Plated Cylinders
There are two types of cylinder bores used on dirt bikes, steel or cast iron sleeves or ones with plating on the aluminum. Most dirt bikes made after 1989 have plated cylinders. You can check a cylinder with a magnet. If it sticks to the bore then it is a sleeve. If it doesn’t stick then it is plated. There are three types of plated cylinders, Kawasaki Electrofusion, hard-chrome, and nickel silicon carbide. There are several variations of the nickel silicon carbide process but the most common trade name is Nikasil. The nickel-based processes have many advantages over hard-chrome, Electrofusion, and sleeving. Nickel attracts oil and is an excellent carrier material for silicon carbide particles, a wear resistant material that carries the load of the piston. This material is electro-plated right on to the aluminum cylinder for the optimum thermal efficiency. Nickel can be honed with diamond stones which leave distinctive peaks and valley scratches in the cylinder wall which retain oil and provide a certain bearing ratio between the running surfaces of the bore. It’s possible to rebuild a plated cylinder by fitting it with a sleeve. However you can expect to pay more for bore maintenance over the life of the bike, and lose thermal efficiency and horsepower. Plated cylinders are harder and last longer than sleeved cylinders. Kawasaki cylinders with the original Electrofusion coating or hard-chromed cylinders can be repaired with nickel plating or sleeving. Steel or cast iron sleeves cannot be nickel plated unless they are separated from the aluminum cylinder. The reason is that the pretreatment for the plating would disintegrate the aluminum. There are four companies that replate cylinders in the USA. The average price to replate a cylinder is about $200.
The Piston
Some unfortunate guys do more damage replacing the piston than the actual wear on the piston! Remove the circlips with a small needle-nose pliers and throw them away. It is a common mistake to reuse circlips, but the cheap spring-steel wire clips will fatigue and break if you install them for a second time.
After removing the circlips, you have to remove the piston pin. Never use a hammer and punch to remove the pin. That will damage the connecting rod and needle bearings. Instead, use one of the pin-extractor tools available from your local franchised motorcycle shop. You can also grasp the piston with one hand and use a 3/8-inch socket extension to push the pin out with your other hand.
Too many people replace their pistons too often. The exact service interval for your bike depends on how hard the bike was run, for how many hours, the quality of the lubrication, and the amount of dirt or other debris in the intake air. Bikes that are run hard with dirty air filters may wear out pistons in only 6 hours, while bikes that are ridden easy with clean filters and adequate fuel octane may last 60 hours.
Measuring the Piston
The best thing to do is measure the piston with a caliper. Digital calipers cost about $100 at industrial tool companies such as Enco or Harbor Freight. A digital caliper is easy to use and gives accurate measurements on the piston diameter and cylinder bore. Measure the widths of the piston (front to back) just above the intake cutaway because this is the widest point of the piston. Check the maximum wear specs in your service manual. Check the piston for detonation marks in the crown, cracks in the skirt, or seizure marks. Look at the underside of the piston crown for a large black spot. The spot is burnt oil deposits that adhered to the piston because the piston crown temperature was too hot. This is an indication that the carb?s main jet needs to be richer.
Letter Designations on Cylinders and Pistons
The Japanese manufacturers use a letter designation system for plated cylinders. They intend for you to order replacement pistons based on the letter designation printed or stamped on the cylinder. This is the reason why they need this type of system. In mass production you can’t guaranty that all parts will be exactly the same size. The size variance is based on an acceptable level of quality. Tool bits become dull, temperatures of machine tools change through production runs, and machine operators have inconsistent performance. The Japanese manufacturers have between two to four different sized pistons and cylinders. Normally labeled A, B, C, and D. If they only had one size, the piston to cylinder wall clearance would vary between .001 to .006 inches. In the standard Japanese alpha labeling system, A denotes the smallest bore or piston size and every letter after that is slightly larger, usually in increments of .0015 inches. The danger is that if you try to put a D piston in an A cylinder the piston to cylinder wall clearance will be so tight that a seizure might occur.
Pro-X Oversize Piston Kits
Pro-X is a marketing company that sells the surplus pistons from the Japanese company ART, which makes all the cast pistons for the Japanese motorcycle manufacturers. These pistons are the same quality as the OEM pistons, and they are available in sizes larger than the alpha pistons available from franchised dealers. Also the Pro-X pistons are usually priced lower than the OEM pistons. If the cylinder bore is slightly worn (up to .005 inches) with only a small area of bare aluminum exposed, you can install a Pro-X oversize piston. The Pro-X pistons are graded oversize in smaller increments than Wiseco pistons, but a wider range than the OEM pistons. For example, Wiseco sizes are .010 inches and Pro-X is .001 inches increments. Before attempting to order a Pro-X piston, you must measure the cylinders bore at the smallest point and allow .002 inches clearance between the piston and cylinder.
Measuring the Ring Gap
The best way to know if the rings are worn is to measure the ring end gap. Put the ring in the cylinder and use the piston to push it down about 1/2 inch from the top evenly spaced. Now use a feeler gauge to measure the width of the ring gap. Normally, the maximum gap is 0.018–0.025 inch.
Cylinder and Exhaust Valve Cleaning
Does your cylinder have burnt-on mud on the outside, heavy brown oil glazing on the cylinder bore, or gooey oil on the exhaust valves? If so, here is a tip for cleaning those parts without flammable cleaners. Go to the grocery store and get a can of aerosol oven cleaner. This stuff is great for cleaning the carbon from the exhaust valves without completely disassembling them. CAUTION: Oven cleaner attacks aluminum, so don?t leave it on the cylinder for more than 20 minutes. Oven cleaner can be used on both steel and plated bores.
The oven cleaner will help loosen the oil glazing on the cylinder walls. Then, you can use a Scotch-Brite pad to hone the cylinder walls in a crisscross pattern. Wear rubber gloves when you use oven cleaner and flush the cylinder afterwards with soapy water. This will neutralize the acid in the oven cleaner and break the molecular bond of the oil, so the debris can be rinsed away. Sleeved (especially Kawasaki cylinder bores) are vulnerable to corrosion after cleaning. Spray some penetrating oil on the cylinder bore to prevent it from rusting.
Caution: Certain types of cylinders corrode quickly after the cleaning process, so spray the bore area with penetrating oil to displace the water.
Honing the Cylinder Bore
Many people have emailed me with questions regarding honing cylinder bores. If you want to buy a hone to deglaze bores or polish off small scratches, then a ball-hone is the best choice. Ball hones are manufactured by Brush Research in Los Angeles, under the brand name Flex-Hone. These hones are available under different labels and they are most easily available from auto parts stores. Buy a size that is 10% smaller than the actual bore size. These hones are available in several different materials and grits but the profile that bests suits both steel and plated cylinders is aluminum oxide 240 grit. A ball hone cannot remove material from the cylinder bore, especially on the hard nickel plated bores. However a ball hone can polish down the peaks of the original hone scratches and increase the bearing ratio. In other words the piston will be touching a greater percentage of the bore. Sometimes that makes the piston wear quicker but if you have to ball hone the bore to remove scratches, it?s a compromise. The one type of hone that you should never use on a two-stroke cylinder is a spring-loaded finger hone. The sharp edges of the stone will snag the port edges and most likely damage the hone and the cylinder.
Top End Assembly
1. Install one of the circlips in the piston with the opening facing away in the 6 or 12 o’clock position.
2. Grease the cylinder-base alignment pins.
3. Set the exhaust valves in the closed position.
4. On cylinders with reed valves, leave the intake port open because you will need to reach in through the port to push the piston-ring ends back in place.
5. The best way to slip the piston into the bottom of the cylinder is to rotate the rings toward one side of the locating pins and squeeze the rings with your middle finger and thumb. That will leave your other hand free to position the cylinder.
6. There are two methods used to assemble to top end. The first method is to attach the piston to the connecting rod and lower the cylinder on to the piston assembly. The second method is to install the piston assembly into the cylinder and lower the cylinder and piston on to the connecting rod. The second method is easier but involves pinning the piston and installing one circlip with a minimum amount of free space.
7. Take care to align the exhaust valve control mechanism as the cylinder is bolted to the crankcases.
Gasket Hygiene
The oven cleaner you used to clean the cylinders will help loosen the old gasket material so you can remove it. Carefully scrape the gasket off with a gasket scraper. Never use a flat screwdriver to remove the old gaskets because the aluminum surfaces of the head, cylinder, and crankcases are easily gouged. If these surfaces are gouged on your engine, they should be draw-filed flat to prevent air or coolant leaks.
Never reuse paper gaskets; always replace them with new gaskets, and spray sealer on the paper gaskets, so they will seal better and will be easier to remove the next time. The new-style steel gaskets can be cleaned and reused a few times, but you?ll need to spray the gasket with a sealer such as Permatex Spray-A-Gasket or copper-coat.
Keep a Logbook
Keep a logbook that tracks the number of riding days and the periodic maintenance. From reviewing the log, you will learn how often you need to service the top end if you record the measurements of the ring gap and the piston diameter. A logbook also gives you greater leverage when you try to sell your used bike for a premium price.
Big Bore Kits
One of the best ways to increase horsepower is to increase displacement by overboring the cylinder. This can be ideal for play or Vet Class riders, where the increased displacement won?t be illegal for your race class. When done right, a big bore kit can give you more power everywhere rather than an increase in only the top or the bottom of the powerband. Such increases are typically more usable and give you more power where you need it.
Piston manufacturers such as Wiseco make oversize piston kits for popular model bikes. These kits boost the displacement of the cylinder to the limit of a racing class or to a larger displacement class, for example: 80cc to 100cc, 125cc to 145cc, 250cc to 265cc or 300cc, and 495cc to 550cc.
The AMA has a limit of overboring any cylinder used in amateur modified classes. The limit is 2 millimeters. Wiseco makes a line of Pro-Lite pistons for this purpose. Normally no head modifications are needed, but cylinders with exhaust valves that operate close to the cylinder bore will need to be trimmed for clearance. Cylinders that use steel head gaskets will require oversize gaskets. Cometic makes 2 millimeter oversize and big bore gasket kits. The process of overboring and electro-plating a cylinder can be a cost effective way to save a cylinder that suffered a top end failure and scored the cylinder wall.
Riders competing in the AMA veteran class can ride a bike with any displacement. Riders competing in hare scrambles and enduro can race the 200cc class with a 125 converted to any displacement. AMA motocross and enduro racers can make the 250cc bikes legal for open class by increasing the displacement a minimum of 15 percent (to 286cc). Wiseco makes 74-millimeter piston kits to convert the popular 250s to 300cc. Be careful if you decide to go with a big bore kit, though. If the overbore is not performed properly, though, it can result in the wrong kind of power or, at worst, a ruined cylinder. When you change the displacement of the cylinder, there are so many factors to consider, such as port time-area, compression ratio, exhaust valves, carb jetting, silencer, and ignition timing. Here is an explanation of what you need to do when planning to overbore a cylinder.
Also, you should at least consult with an expert before tackling a big bore kit. To get the most from an overbored engine, you need to make sure the carburetion, exhaust, porting, and timing are all adjusted to suit the larger bore.
Port-Time Area
The term port-time area refers to the size and flow range of the intake and exhaust ports, relative to rpm. The ports enter the cylinder bore at angles. When the cylinder is over-bored the transfer ports become lower and wider. The same thing happens to the exhaust port. This effectively retards the port timing and reduces the total degrees of duration. When the displacement of the engine increases, so does the demand for more port-time-area.
If you just overbored and plated a cylinder, it would have much more low-end power than stock but the top-end power would suffer. Normally tuners have to adjust the ports to suit the demands of the larger engine displacement. The proper dimensions for the ports can be calculated using a computer program from Two-Stroke Racing (TSR) www.tsrsoftware.com The program “PORTTIME” enables tuners with limited math skills to run strings of formulas for determining the optimum dimensions of the ports. Generally speaking, if the ports in the overbored cylinder were raised to the same heights as the stock cylinder, that would make the port timing sufficient to run with stock or aftermarket exhaust systems.
Cylinder Head
After overboring the cylinder, the head?s dimensions must be changed to suit the larger piston. First, the head?s bore must be enlarged to the finished bore size. Then, the squish-band deck height must be set to the proper installed squish clearance. The larger bore size will increase the squish turbulence, so the head?s squish band may have to be narrowed. The volume of the head must be increased to suit the change in cylinder displacement. Otherwise, the engine will run flat at high rpm or ping in the midrange from detonation.
Exhaust Valves
When the bore size is increased, the exhaust valve-to-piston clearance must be checked and adjusted. This pertains to the types of exhaust valves that operate within close proximity of the piston. If the exhaust valves aren?t modified, the piston could strike the valves and cause serious engine damage. The normal clearance between the exhaust valves and the piston should be at least .030 inches or .75 millimeters
Carburetor
The larger the ratio between the piston?s diameter and the carb?s size, the higher the intake velocity. Overbored cylinders produce higher intake velocity which draws more fuel through the carb. Of course a larger engine will need more fuel. Normally when you overbore an engine 15-20%, the slow jet will need to be richened and the main jet will need to be leaned. Start with the stock jetting and make adjustments after you ride the bike.
Ignition Timing
The ignition timing has a minimal affect on the poweband. Retarding the timing has the affect of reducing the hit of the powerband in the midrange and extending the top end over rev. “Overrev” is a slang term that describes the useable length of the powerband at high rpm.
The scientific reason for the shift of the powerband to extremely high rpm, is because the temperature in the pipe increases with the retarded timing, and that enables the pipe?s tuned length to be more synchronous with the piston speed and port timing of the cylinder.
Advancing the timing has the affect of increasing the midrange hit of the powerband, but makes the power flatten out at high rpm. The reason is that the relatively long spark lead time enables for a greater pressure rise in the cylinder before the piston reaches TDC. This produces more torque in the midrange but the high pressure contributes to pumping losses at extremly high rpm.
Pipe and Silencer
Because only the bore size is changed, you won?t need a longer pipe—only one with a larger center section. FMF?s line of Fatty pipes work great on engines that have been overbored.
Head Gasket
The head gasket will need to have the bore diameter increased to the dimension of the new piston. If the head gasket overlaps into the cylinder bore more than one millimeters on each side, it could contact the piston or be susceptible to pressure blowouts.
10 TIPS FOR REBUILDING A TWO-STROKE TOP END
1) Before you disassemble your engine, power-wash the engine and the rest of the vehicle. That will reduce the risk of dirt and debris falling into the engine. Once you remove the cylinder, stuff a clean rag down into the crankcases.
2) The cylinder and head use alignment pins to hold them straight in position from the crankcases on up. The pins make it difficult to remove the cylinder from the cases and the head from the cylinder. Sometimes the steel alignment pins corrode into the aluminum engine components. Try spraying penetrating-oil down the mounting studs before attempting to remove the cylinder and head. Never use a flat-blade screwdriver, chisel, or metal hammer to remove the cylinder. Instead use this technique; buy a lead-shot plastic mallet, swing it at a 45-degree angle upwards against the sides of the cylinder. Alternate from left to right, hitting the sides of the cylinder to separate it from the cases evenly. Clean the steel alignment pins with steel wool and penetrating-oil. Examine the pins closely. If they are deformed in shape, they won?t allow the engine parts to bolt together tightly. This can cause a dangerous air leak or a coolant leak. The pins are cheap at about $2 each. Replace them if they?re rusty or deformed.
3) Never re-use old gaskets. Remove them with a razor blade or gasket scraper. Don?t use a drill-driven steel wool type pad to remove old gaskets because they can remove aluminum from the cylinder and head. That will cause a gasket to leak.
4) Always check the ring end gap on a new ring by placing it in the cylinder between the head gasket surface and the exhaust port. The gap should be between .012 to .024 inches.
5) Always install the circlips with the opening facing straight up or down, that way inertia will hold it tight into the clip groove. Place one clip in the groove before installing the piston on the connecting rod. Its easier to install a clip with the piston in your hand rather than on the rod. There also less chance that you?ll drop the circlip in the crankcases.
5) Always install the rings on the piston with the markings facing up. Coat the rings with pre-mix oil so they can slide in the groove when trying to install the piston in the cylinder.
6) Always install the piston on the connecting rod with the arrow on the piston crown facing towards the exhaust port.
7) The traditional way to assemble the top end is to install the piston assembly on the connecting rod, compress the rings, and slide the cylinder over the piston. That can be difficult with larger bore cylinders, or if you?re working by yourself. Try this method instead. Install one circlip in the piston, install the piston into the cylinder with the pin hole exposed, install the piston pin through one side of the piston, position the cylinder over the connecting rod and push the piston pin through until it bottoms against the circlip, install the other circlip. It only takes two hands to install the top end using this manor and there is less chance that you?ll damage the rings by twisting the cylinder upon installation.
8) On cylinders with reed valves and large oval intake ports, take care when installing the piston assembly in the cylinder because the rings are likely to squeeze out of the ring grooves. Use a flat-blade screwdriver to gently push the rings back in the grooves so the piston assembly can pass by the intake port.
9) For steel head gaskets, place the round side of the “bump” facing up. Don?t use liquid gasket sealer; use aerosol spray adhesive types instead. For hybrid fiber/steel ring head gaskets, place the wide side of the steel rings facing down.
10) When you initially start the engine after a rebuild, manipulate the choke to keep the engine rpm relatively low. Once the engine is warm enough to take it off choke, drive the vehicle around on flat hard ground. Keep it under 2/3 throttle for the first 30 minutes. Two common myths for proper engine break-in are; 1) Set the engine at a fast idle, stationary on a stand. 2) Add extra pre-mix oil to the fuel. When the engine is on a stand it doesn?t have any air passing through the radiator and it is in danger of running too hot. When you add extra oil to the fuel you are effectively leaning the carb jetting. This can make the engine run hotter and seize.
Top-End Seized After Rebuild
Question: I trail ride a 1989 YZ250. Last winter, I rebuilt the top end after reading your article in Dirt Rider. The bore was so worn that I had to skip to a one millimeter-oversize piston kit, just so the bore job would clean up a severely worn spot below the intake port. After I rebuilt the top end, I cycled the engine by letting it idle for three 15-minute sessions with adequate cool-down periods in between. When I first rode the bike, I heard some detonation noises but didn?t think it was a serious problem, until it seized. What could be wrong?
Answer: Your problem is simple. When a cylinder is overbored, the displacement is increased and that boosts the compression ratio. Whenever a cylinder is overbored more than 0.010 inches or 0.25mms the cylinder-head diameter must be enlarged to the new bore size. Otherwise, the piston could contact the head or the edge of the head surface that extends into the bore could cause a hot-spot and pre-ignition. Also, the cylinder head?s squish band must be narrowed by enlarging the combustion-chamber bowl. This also serves to increase the head?s volume, thereby lowering the compression ratio. This work must be performed on a lathe by a qualified tuner or machinist. Average cost of this service is $50
Base Gasket Seeping
Question: I recently rebuilt the top end on my 1991 CR250. I was being as careful as I could be while taking the cylinder off, but the dowels were fused in pretty good and I had to pry it. Needless to say, I gouged the case a bit. I smoothed it out with sandpaper and reassembled the engine. The bike runs great, but a little oil seeps out of the cylinder-to-case mating surface. I assume this is transmission oil? Would it be OK to use something like a thin layer of Permatex Blue or Yamabond here? Would this make it even more difficult to remove the cylinder in the future? Should I just let it alone? The best price I could find on a new left side case was $215 and I?m sure it would be a lot of work and a lot of replacing gaskets along the way. Am I out of luck?
Answer: Air leaks can be very dangerous because the engine could rev independent of the throttle. An inexpensive way to fix your bike?s problem is to draw-file the cylinder base and the crankcases. Then apply a thin coating of Yamabond or any other brand of non-drying sealer to both sides of the base gasket. The best technique for removing cylinders is to tap up on each side of the cylinder with a lead-shot plastic mallet. Remember to put a dab of grease on the cylinder-base dowel pins.
Two-Stroke Exhaust Valves
Three words sum up exhaust valve maintenance: spoogey, gooey, and grungy. If two-stroke exhaust valves didn?t have such a dramatic effect on the engine?s powerband, I?m sure mechanics would remove them and beat them bits with a hammer in frustration because there is little information given by the manufacturers on how to diagnose and repair the exhaust valve systems on well-used dirt bikes. This section is a guide to the characteristic mechanical problems that occur to the exhaust valve systems of dirt bikes. Plus we?ll give you some tips on how to re-time exhaust valve systems.
How Exhaust Valves Work
An exhaust valve system is designed to increase the engine?s low-end and midrange power. There are three different designs of exhaust valve systems. The first-generation design uses a variable-volume chamber mounted to the head pipe to change the tuned length of the head pipe. A butterfly valve is used to separate the surge chamber and the head pipe. At low rpm, the valve is open to allow the pressure waves in the pipe to travel into the surge chamber and effectively lengthen the pipe and reduce the pressure wave?s magnitude when it returns to the exhaust port. These systems were primitive and not very effective on 125cc dirt bikes. Honda and Suzuki used this type of exhaust valve system in the mid to late 1980s.
The second-generation design features valves that control the effective stroke and the time-area of the exhaust port. These valves are fitted to the sub-exhaust ports and the main exhaust port. The main exhaust-port valves operate within close proximity to the piston to control the effective stroke of the engine. The effective stroke is defined as the distance from TDC to when the exhaust port opens. At low rpm, the engine needs a long effective stroke, which results in a high compression ratio. At high rpm, the engine needs a shorter effective stroke, longer exhaust duration, greater time-area, and a lower compression ratio. Yamaha used this system starting in 1982 on the YZ250. Honda?s HPP system is similar and was used on the 1986–91 CR250 and 1990 to current-model CR125.
The third generation of exhaust valve systems attempts to change the exhaust-port velocity, effective stroke, exhaust-gas temperature, and the pressure of the compression wave. Yamaha and Suzuki started using these systems on their 125s in 1995. Both companies employed a venting system to the outside atmosphere. This is very complex because they are attempting to affect the temperature and pressure of the returning compression wave to synchronize it with the piston speed. The exhaust-gas velocity and the effective stroke are controlled by two oval wedge valves that enter the exhaust port at a 45-degree angle. The wedge valves partially block the exhaust port, thereby boosting the gas velocity. Kawasaki?s KIPS system uses wedge valves in the main exhaust port to control the effective stroke, drum valves in the sub-exhaust ports to control the time-area, and a surge chamber to absorb the excess compression-wave pressure at low rpm.
The exhaust valves are opened and closed by a centrifugal governor mechanism. The governor is mounted under the right side cover and is gear-driven by the crankshaft. As the engine rpm increases, the governor spins, thereby increasing the angular momentum of the four steel balls encased in the governor. The steel balls fit into an angled ramp-and-cup arrangement. A spring is used to provide tension on the steel balls. When the momentum of the steel balls overcomes the spring?s tension, and the balls force their way up the angled ramp. A spool attached to the ramp, enabling it to change its linear position with changes in rpm, and the spool is attached to a linkage system that operates the exhaust valves in the cylinder. Factory race teams have different combinations of springs, ramps, and balls to tune the exhaust valve operation and enhance the powerband.
Exhaust Valve Tips and Tuning
Although exhaust valves use the same essential principles, the implementation is different with each manufacturer. Also, each type has its own flaws and fixes. The list below gives you tips on how to install and service the most common exhaust valves, as well as some tuning tips
Yamaha POWERVALVE
Yamaha was the first motorcycle manufacturer to adapt exhaust valves to two-stroke motorcycle engines. Yamaha?s simple design of a cylindrical valve that rotates 1/4 turn to vary the height of the exhaust port requires little maintenance. This system was used on the YZ250 from 1982-98, and on the YZ125 from 1983-93. Occasionally, you have to replace the seals and O-rings to prevent exhaust oil from drooling out of the side if the cylinder. In 1989, Yamaha added a stop plate to limit the travel of the power valve, primarily so mechanics couldn?t install the valve in the wrong position. The stop plate is located on the left side of the cylinder. The valve has a small tab that bumps up against the stop plate to limit the fully open and closed position of the valve. This design enabled Yamaha to position the valve closer to the piston to make it more effective at varying the exhaust-port timing. Unfortunately, the soft-aluminum tab on the valve gets worn, allowing the valve to rotate farther in the fully closed position. Eventually, (after about three years? use) the tab wears enough so the valve strikes the piston, causing damage to the piston. Yamaha?s exhaust valve is cheap to replace. I recommend replacing the valve when the tab wears more than 0.030 inch (0.7mm).
In 1994 Yamaha changed the engine design of the YZ125 and included the next generation of exhaust valves. This system used two oval-shaped wedge valves, positioned at a 45-degree angle over the exhaust port. This system was similar to the one employed by Suzuki. Yamaha experimented with resonator cavity volume, and vents for pressure bleed off and temperature control. Overall this is a very reliable system. Occasionally the pins that fit through the ends of the valve to interface with the actuator lever vibrate out causing the valve to strike the piston. Those pins are a press fit but you can add some Loctite Instant Adhesive to the pins for added protection. One problem that Yamaha is concerned with is high rpm valve flutter. They’ve added springs to the valves to control the flutter but future innovations could include a positive seal between the valve and the cylinders’ valve pocket.
In 1999 Yamaha redesigned the YZ250 engine and exhaust valve system. This model features a powervalve that marks a significant design change, from the company that pioneered the use of exhaust valves on two-stroke engines. Looking more like a Rube Goldberg device, the new powervalve has separate valves for the main (center) and sub-exhaust ports (sides). The whole assembly is controlled by one actuating rod, but the side valves open after the main exhaust valve. The side valves are controlled by two wedge-shaped ramps but you can bet that the factory teams are experimenting with them. The stop plate of the center valve tends to crack, allowing the valve to contact the piston. Look for cracks in the plate richt around the two retaining bolts.
Top-end rebuilding is the most frequent and costly service routine on two-stroke dirt bikes. Every year, dirt bike riders waste loads of money on top-end parts that didn’t need to be replaced, or make costly mistakes while performing repairs. This section will give you the dos and don’ts to easy top-end rebuilding, plus some tips that aren’t printed in your factory service manual.
Before You Start
Thoroughly wash your bike because dirt stuck to the underside of the top frame tube could break loose when servicing and fall into the engine! Use a stiff plastic brush and hot soapy water to clean off the grit and grime around the base of the cylinder, on the carburetor and intake boot, and especially underneath the top frame rail. Degreaser can be used on metal surfaces, but take care not to leave it on rubber or gasket surfaces.
Tools
You’ll need at least some 3/8-inch-drive metric sockets and box wrenches (open-end wrenches can round off the edges on the cylinder or head nuts, and shouldn’t be used for top-end rebuilding), a needle-nose pliers for removing circlips, and a gasket tool to scrape the old gaskets away. For soft tools, get some shop towels, aerosol oven cleaner, a Scotch-Brite pad, a locking agent such as Loctite, a gasket scraper, a brush, and a bucket of soapy water. Regarding measuring tools, you’ll need a compression tester, a feeler gauge, and a digital vernier caliper.
Compression Testing
A compression tester is a useful diagnostic tool, and readily available from Sears or auto parts stores. Buy the threaded type and make sure the kit comes with an adapter that matches the spark plug threads of your engine. Performing a compression test is simple. Start by removing the spark plug, thread in the adapter, and hold the throttle wide open and the kill button on. This will prevent any spark and enable the engine to draw in maximum airflow. Then kick-start the engine several times until the needle on the pressure gauge peaks. The pressure reading depends on two main factors; the compression ratio and the altitude at which the engine is being tested. The compression ratio will also depend on if the engine is equipped with exhaust valves and their condition. When the exhaust valves are in the closed position the compression ratio will be greater than if the valves are carbon-seized in the open position. The difference may yield a pressure reading 25 psi. The quality of compression testers varies greatly. The main thing that a compression tester can identify is a change in condition. Whenever you rebuild the top end, take a compression pressure reading and mark it down. When the pressure changes 20% check the condition of the piston and rings. Pistons usually last twice as long as rings.
Crankcase Pressure Testing
The crankcase of a two-stroke engine is sealed from the tranny. It’s important that the two crankshaft seals be in optimum condition. One side of the crankshaft uses a dry seal and the other a wet seal. The dry seal runs on the magneto side and the wet seal runs in oil on the tranny side. When the dry seal wears, the crankcase sucks in hot air, causing the mixture to run lean and overheat the engine. When the wet seal wears, the crankcase sucks in tranny oil, causing the engine run rich and eventually wet-foul the spark plug.
A crankcase pressure test involves the use of a vacuum pump with spark plug adapter, and rubber plugs to block off the intake and exhaust manifolds of the cylinder. The piston must be positioned at BDC to allow the transfer ports to be wide open linking the bore and the crankcase. The hand-pump produces vacuum pressure up to a standard setting of 5 psi. The normal bleed-down pressure loss is 1 psi per minute. Cylinders with complicated exhaust valve systems can be difficult to block-off air leaks, and harder to test. Crankcase pressure testing kits are available from Motion Pro.
If I suspect that an engine has an air leak in the crankcases, I do a visual test. Start by power washing the engine clean. Then remove the magneto cover. Spray the magneto clean with an aerosol can of brake cleaner. Make sure to use a non-chlorinated type of cleaner (green colored can). Now spray baby powder to all the suspect areas of the engine. Spray the powder on the crankcase around the magneto, at the crankcase seam line, the cylinder base, and the reed valve. Run the engine for a while, the white baby powder will highlight any fluid or air leaks on the engine. The baby powder test is much better than the alternative test of blowing raw propane gas at different areas of a running engine and listening for a change in the idle rpm. That is dangerous because it involves flammable gas and a hot engine with random electrical shorts.
Maintenance and Inspection
A thorough top-end rebuild requires removing the reed valve, cylinder head, and cylinder. You should tear down your top end periodically and inspect the reed valve, cylinder head, cylinder, piston, and so on. Use the following chart to determine when you should tear down your bike:
Displacement: 80cc 125cc 250cc 500cc
Tear down after: 20 hours 30 hours 40 hours 60 hours
Note that air-cooled bikes should be inspected more frequently. Also, you may want to inspect more often if you are riding in fine sand or lots of mud. When you tear down the engine, inspect each system and look for the following trouble signs.
Reed Valve
Check the reed petals for open gaps between the sealing surfaces. In time, the reed petals lose their spring tension, and the back-flow can cause a flat-spot in the throttle response. Stock nylon reeds tend to split at the edges on bikes that are constantly over-revved. Expert riders find that carbon fiber reeds last much longer.
Cylinder Head
Check the head at the edge of the chamber for erosion marks—a sign that the head gasket was leaking. If the head or top edge of the cylinder is eroded, it must be turned on a lathe to be resurfaced.
Cylinder
All cylinder bases use aligning (dowel. pins around two of the cylinder base studs. These pins are made of steel, and after heavy power washing, they get corroded. That makes it difficult to remove the cylinder from the crankcases. Never use a pry bar! That will damage the cylinder. Instead use a plastic mallet to hit upward on the sides of the cylinder at a 45-degree angle. Alternate from left to right sides so the cylinder lifts up evenly. After you remove the cylinder, stuff a shop towel into the open crankcases to prevent debris from entering the engine.
The Different Types of Steel-Lined and Plated Cylinders
There are two types of cylinder bores used on dirt bikes, steel or cast iron sleeves or ones with plating on the aluminum. Most dirt bikes made after 1989 have plated cylinders. You can check a cylinder with a magnet. If it sticks to the bore then it is a sleeve. If it doesn’t stick then it is plated. There are three types of plated cylinders, Kawasaki Electrofusion, hard-chrome, and nickel silicon carbide. There are several variations of the nickel silicon carbide process but the most common trade name is Nikasil. The nickel-based processes have many advantages over hard-chrome, Electrofusion, and sleeving. Nickel attracts oil and is an excellent carrier material for silicon carbide particles, a wear resistant material that carries the load of the piston. This material is electro-plated right on to the aluminum cylinder for the optimum thermal efficiency. Nickel can be honed with diamond stones which leave distinctive peaks and valley scratches in the cylinder wall which retain oil and provide a certain bearing ratio between the running surfaces of the bore. It’s possible to rebuild a plated cylinder by fitting it with a sleeve. However you can expect to pay more for bore maintenance over the life of the bike, and lose thermal efficiency and horsepower. Plated cylinders are harder and last longer than sleeved cylinders. Kawasaki cylinders with the original Electrofusion coating or hard-chromed cylinders can be repaired with nickel plating or sleeving. Steel or cast iron sleeves cannot be nickel plated unless they are separated from the aluminum cylinder. The reason is that the pretreatment for the plating would disintegrate the aluminum. There are four companies that replate cylinders in the USA. The average price to replate a cylinder is about $200.
The Piston
Some unfortunate guys do more damage replacing the piston than the actual wear on the piston! Remove the circlips with a small needle-nose pliers and throw them away. It is a common mistake to reuse circlips, but the cheap spring-steel wire clips will fatigue and break if you install them for a second time.
After removing the circlips, you have to remove the piston pin. Never use a hammer and punch to remove the pin. That will damage the connecting rod and needle bearings. Instead, use one of the pin-extractor tools available from your local franchised motorcycle shop. You can also grasp the piston with one hand and use a 3/8-inch socket extension to push the pin out with your other hand.
Too many people replace their pistons too often. The exact service interval for your bike depends on how hard the bike was run, for how many hours, the quality of the lubrication, and the amount of dirt or other debris in the intake air. Bikes that are run hard with dirty air filters may wear out pistons in only 6 hours, while bikes that are ridden easy with clean filters and adequate fuel octane may last 60 hours.
Measuring the Piston
The best thing to do is measure the piston with a caliper. Digital calipers cost about $100 at industrial tool companies such as Enco or Harbor Freight. A digital caliper is easy to use and gives accurate measurements on the piston diameter and cylinder bore. Measure the widths of the piston (front to back) just above the intake cutaway because this is the widest point of the piston. Check the maximum wear specs in your service manual. Check the piston for detonation marks in the crown, cracks in the skirt, or seizure marks. Look at the underside of the piston crown for a large black spot. The spot is burnt oil deposits that adhered to the piston because the piston crown temperature was too hot. This is an indication that the carb?s main jet needs to be richer.
Letter Designations on Cylinders and Pistons
The Japanese manufacturers use a letter designation system for plated cylinders. They intend for you to order replacement pistons based on the letter designation printed or stamped on the cylinder. This is the reason why they need this type of system. In mass production you can’t guaranty that all parts will be exactly the same size. The size variance is based on an acceptable level of quality. Tool bits become dull, temperatures of machine tools change through production runs, and machine operators have inconsistent performance. The Japanese manufacturers have between two to four different sized pistons and cylinders. Normally labeled A, B, C, and D. If they only had one size, the piston to cylinder wall clearance would vary between .001 to .006 inches. In the standard Japanese alpha labeling system, A denotes the smallest bore or piston size and every letter after that is slightly larger, usually in increments of .0015 inches. The danger is that if you try to put a D piston in an A cylinder the piston to cylinder wall clearance will be so tight that a seizure might occur.
Pro-X Oversize Piston Kits
Pro-X is a marketing company that sells the surplus pistons from the Japanese company ART, which makes all the cast pistons for the Japanese motorcycle manufacturers. These pistons are the same quality as the OEM pistons, and they are available in sizes larger than the alpha pistons available from franchised dealers. Also the Pro-X pistons are usually priced lower than the OEM pistons. If the cylinder bore is slightly worn (up to .005 inches) with only a small area of bare aluminum exposed, you can install a Pro-X oversize piston. The Pro-X pistons are graded oversize in smaller increments than Wiseco pistons, but a wider range than the OEM pistons. For example, Wiseco sizes are .010 inches and Pro-X is .001 inches increments. Before attempting to order a Pro-X piston, you must measure the cylinders bore at the smallest point and allow .002 inches clearance between the piston and cylinder.
Measuring the Ring Gap
The best way to know if the rings are worn is to measure the ring end gap. Put the ring in the cylinder and use the piston to push it down about 1/2 inch from the top evenly spaced. Now use a feeler gauge to measure the width of the ring gap. Normally, the maximum gap is 0.018–0.025 inch.
Cylinder and Exhaust Valve Cleaning
Does your cylinder have burnt-on mud on the outside, heavy brown oil glazing on the cylinder bore, or gooey oil on the exhaust valves? If so, here is a tip for cleaning those parts without flammable cleaners. Go to the grocery store and get a can of aerosol oven cleaner. This stuff is great for cleaning the carbon from the exhaust valves without completely disassembling them. CAUTION: Oven cleaner attacks aluminum, so don?t leave it on the cylinder for more than 20 minutes. Oven cleaner can be used on both steel and plated bores.
The oven cleaner will help loosen the oil glazing on the cylinder walls. Then, you can use a Scotch-Brite pad to hone the cylinder walls in a crisscross pattern. Wear rubber gloves when you use oven cleaner and flush the cylinder afterwards with soapy water. This will neutralize the acid in the oven cleaner and break the molecular bond of the oil, so the debris can be rinsed away. Sleeved (especially Kawasaki cylinder bores) are vulnerable to corrosion after cleaning. Spray some penetrating oil on the cylinder bore to prevent it from rusting.
Caution: Certain types of cylinders corrode quickly after the cleaning process, so spray the bore area with penetrating oil to displace the water.
Honing the Cylinder Bore
Many people have emailed me with questions regarding honing cylinder bores. If you want to buy a hone to deglaze bores or polish off small scratches, then a ball-hone is the best choice. Ball hones are manufactured by Brush Research in Los Angeles, under the brand name Flex-Hone. These hones are available under different labels and they are most easily available from auto parts stores. Buy a size that is 10% smaller than the actual bore size. These hones are available in several different materials and grits but the profile that bests suits both steel and plated cylinders is aluminum oxide 240 grit. A ball hone cannot remove material from the cylinder bore, especially on the hard nickel plated bores. However a ball hone can polish down the peaks of the original hone scratches and increase the bearing ratio. In other words the piston will be touching a greater percentage of the bore. Sometimes that makes the piston wear quicker but if you have to ball hone the bore to remove scratches, it?s a compromise. The one type of hone that you should never use on a two-stroke cylinder is a spring-loaded finger hone. The sharp edges of the stone will snag the port edges and most likely damage the hone and the cylinder.
Top End Assembly
1. Install one of the circlips in the piston with the opening facing away in the 6 or 12 o’clock position.
2. Grease the cylinder-base alignment pins.
3. Set the exhaust valves in the closed position.
4. On cylinders with reed valves, leave the intake port open because you will need to reach in through the port to push the piston-ring ends back in place.
5. The best way to slip the piston into the bottom of the cylinder is to rotate the rings toward one side of the locating pins and squeeze the rings with your middle finger and thumb. That will leave your other hand free to position the cylinder.
6. There are two methods used to assemble to top end. The first method is to attach the piston to the connecting rod and lower the cylinder on to the piston assembly. The second method is to install the piston assembly into the cylinder and lower the cylinder and piston on to the connecting rod. The second method is easier but involves pinning the piston and installing one circlip with a minimum amount of free space.
7. Take care to align the exhaust valve control mechanism as the cylinder is bolted to the crankcases.
Gasket Hygiene
The oven cleaner you used to clean the cylinders will help loosen the old gasket material so you can remove it. Carefully scrape the gasket off with a gasket scraper. Never use a flat screwdriver to remove the old gaskets because the aluminum surfaces of the head, cylinder, and crankcases are easily gouged. If these surfaces are gouged on your engine, they should be draw-filed flat to prevent air or coolant leaks.
Never reuse paper gaskets; always replace them with new gaskets, and spray sealer on the paper gaskets, so they will seal better and will be easier to remove the next time. The new-style steel gaskets can be cleaned and reused a few times, but you?ll need to spray the gasket with a sealer such as Permatex Spray-A-Gasket or copper-coat.
Keep a Logbook
Keep a logbook that tracks the number of riding days and the periodic maintenance. From reviewing the log, you will learn how often you need to service the top end if you record the measurements of the ring gap and the piston diameter. A logbook also gives you greater leverage when you try to sell your used bike for a premium price.
Big Bore Kits
One of the best ways to increase horsepower is to increase displacement by overboring the cylinder. This can be ideal for play or Vet Class riders, where the increased displacement won?t be illegal for your race class. When done right, a big bore kit can give you more power everywhere rather than an increase in only the top or the bottom of the powerband. Such increases are typically more usable and give you more power where you need it.
Piston manufacturers such as Wiseco make oversize piston kits for popular model bikes. These kits boost the displacement of the cylinder to the limit of a racing class or to a larger displacement class, for example: 80cc to 100cc, 125cc to 145cc, 250cc to 265cc or 300cc, and 495cc to 550cc.
The AMA has a limit of overboring any cylinder used in amateur modified classes. The limit is 2 millimeters. Wiseco makes a line of Pro-Lite pistons for this purpose. Normally no head modifications are needed, but cylinders with exhaust valves that operate close to the cylinder bore will need to be trimmed for clearance. Cylinders that use steel head gaskets will require oversize gaskets. Cometic makes 2 millimeter oversize and big bore gasket kits. The process of overboring and electro-plating a cylinder can be a cost effective way to save a cylinder that suffered a top end failure and scored the cylinder wall.
Riders competing in the AMA veteran class can ride a bike with any displacement. Riders competing in hare scrambles and enduro can race the 200cc class with a 125 converted to any displacement. AMA motocross and enduro racers can make the 250cc bikes legal for open class by increasing the displacement a minimum of 15 percent (to 286cc). Wiseco makes 74-millimeter piston kits to convert the popular 250s to 300cc. Be careful if you decide to go with a big bore kit, though. If the overbore is not performed properly, though, it can result in the wrong kind of power or, at worst, a ruined cylinder. When you change the displacement of the cylinder, there are so many factors to consider, such as port time-area, compression ratio, exhaust valves, carb jetting, silencer, and ignition timing. Here is an explanation of what you need to do when planning to overbore a cylinder.
Also, you should at least consult with an expert before tackling a big bore kit. To get the most from an overbored engine, you need to make sure the carburetion, exhaust, porting, and timing are all adjusted to suit the larger bore.
Port-Time Area
The term port-time area refers to the size and flow range of the intake and exhaust ports, relative to rpm. The ports enter the cylinder bore at angles. When the cylinder is over-bored the transfer ports become lower and wider. The same thing happens to the exhaust port. This effectively retards the port timing and reduces the total degrees of duration. When the displacement of the engine increases, so does the demand for more port-time-area.
If you just overbored and plated a cylinder, it would have much more low-end power than stock but the top-end power would suffer. Normally tuners have to adjust the ports to suit the demands of the larger engine displacement. The proper dimensions for the ports can be calculated using a computer program from Two-Stroke Racing (TSR) www.tsrsoftware.com The program “PORTTIME” enables tuners with limited math skills to run strings of formulas for determining the optimum dimensions of the ports. Generally speaking, if the ports in the overbored cylinder were raised to the same heights as the stock cylinder, that would make the port timing sufficient to run with stock or aftermarket exhaust systems.
Cylinder Head
After overboring the cylinder, the head?s dimensions must be changed to suit the larger piston. First, the head?s bore must be enlarged to the finished bore size. Then, the squish-band deck height must be set to the proper installed squish clearance. The larger bore size will increase the squish turbulence, so the head?s squish band may have to be narrowed. The volume of the head must be increased to suit the change in cylinder displacement. Otherwise, the engine will run flat at high rpm or ping in the midrange from detonation.
Exhaust Valves
When the bore size is increased, the exhaust valve-to-piston clearance must be checked and adjusted. This pertains to the types of exhaust valves that operate within close proximity of the piston. If the exhaust valves aren?t modified, the piston could strike the valves and cause serious engine damage. The normal clearance between the exhaust valves and the piston should be at least .030 inches or .75 millimeters
Carburetor
The larger the ratio between the piston?s diameter and the carb?s size, the higher the intake velocity. Overbored cylinders produce higher intake velocity which draws more fuel through the carb. Of course a larger engine will need more fuel. Normally when you overbore an engine 15-20%, the slow jet will need to be richened and the main jet will need to be leaned. Start with the stock jetting and make adjustments after you ride the bike.
Ignition Timing
The ignition timing has a minimal affect on the poweband. Retarding the timing has the affect of reducing the hit of the powerband in the midrange and extending the top end over rev. “Overrev” is a slang term that describes the useable length of the powerband at high rpm.
The scientific reason for the shift of the powerband to extremely high rpm, is because the temperature in the pipe increases with the retarded timing, and that enables the pipe?s tuned length to be more synchronous with the piston speed and port timing of the cylinder.
Advancing the timing has the affect of increasing the midrange hit of the powerband, but makes the power flatten out at high rpm. The reason is that the relatively long spark lead time enables for a greater pressure rise in the cylinder before the piston reaches TDC. This produces more torque in the midrange but the high pressure contributes to pumping losses at extremly high rpm.
Pipe and Silencer
Because only the bore size is changed, you won?t need a longer pipe—only one with a larger center section. FMF?s line of Fatty pipes work great on engines that have been overbored.
Head Gasket
The head gasket will need to have the bore diameter increased to the dimension of the new piston. If the head gasket overlaps into the cylinder bore more than one millimeters on each side, it could contact the piston or be susceptible to pressure blowouts.
10 TIPS FOR REBUILDING A TWO-STROKE TOP END
1) Before you disassemble your engine, power-wash the engine and the rest of the vehicle. That will reduce the risk of dirt and debris falling into the engine. Once you remove the cylinder, stuff a clean rag down into the crankcases.
2) The cylinder and head use alignment pins to hold them straight in position from the crankcases on up. The pins make it difficult to remove the cylinder from the cases and the head from the cylinder. Sometimes the steel alignment pins corrode into the aluminum engine components. Try spraying penetrating-oil down the mounting studs before attempting to remove the cylinder and head. Never use a flat-blade screwdriver, chisel, or metal hammer to remove the cylinder. Instead use this technique; buy a lead-shot plastic mallet, swing it at a 45-degree angle upwards against the sides of the cylinder. Alternate from left to right, hitting the sides of the cylinder to separate it from the cases evenly. Clean the steel alignment pins with steel wool and penetrating-oil. Examine the pins closely. If they are deformed in shape, they won?t allow the engine parts to bolt together tightly. This can cause a dangerous air leak or a coolant leak. The pins are cheap at about $2 each. Replace them if they?re rusty or deformed.
3) Never re-use old gaskets. Remove them with a razor blade or gasket scraper. Don?t use a drill-driven steel wool type pad to remove old gaskets because they can remove aluminum from the cylinder and head. That will cause a gasket to leak.
4) Always check the ring end gap on a new ring by placing it in the cylinder between the head gasket surface and the exhaust port. The gap should be between .012 to .024 inches.
5) Always install the circlips with the opening facing straight up or down, that way inertia will hold it tight into the clip groove. Place one clip in the groove before installing the piston on the connecting rod. Its easier to install a clip with the piston in your hand rather than on the rod. There also less chance that you?ll drop the circlip in the crankcases.
5) Always install the rings on the piston with the markings facing up. Coat the rings with pre-mix oil so they can slide in the groove when trying to install the piston in the cylinder.
6) Always install the piston on the connecting rod with the arrow on the piston crown facing towards the exhaust port.
7) The traditional way to assemble the top end is to install the piston assembly on the connecting rod, compress the rings, and slide the cylinder over the piston. That can be difficult with larger bore cylinders, or if you?re working by yourself. Try this method instead. Install one circlip in the piston, install the piston into the cylinder with the pin hole exposed, install the piston pin through one side of the piston, position the cylinder over the connecting rod and push the piston pin through until it bottoms against the circlip, install the other circlip. It only takes two hands to install the top end using this manor and there is less chance that you?ll damage the rings by twisting the cylinder upon installation.
8) On cylinders with reed valves and large oval intake ports, take care when installing the piston assembly in the cylinder because the rings are likely to squeeze out of the ring grooves. Use a flat-blade screwdriver to gently push the rings back in the grooves so the piston assembly can pass by the intake port.
9) For steel head gaskets, place the round side of the “bump” facing up. Don?t use liquid gasket sealer; use aerosol spray adhesive types instead. For hybrid fiber/steel ring head gaskets, place the wide side of the steel rings facing down.
10) When you initially start the engine after a rebuild, manipulate the choke to keep the engine rpm relatively low. Once the engine is warm enough to take it off choke, drive the vehicle around on flat hard ground. Keep it under 2/3 throttle for the first 30 minutes. Two common myths for proper engine break-in are; 1) Set the engine at a fast idle, stationary on a stand. 2) Add extra pre-mix oil to the fuel. When the engine is on a stand it doesn?t have any air passing through the radiator and it is in danger of running too hot. When you add extra oil to the fuel you are effectively leaning the carb jetting. This can make the engine run hotter and seize.
Top-End Seized After Rebuild
Question: I trail ride a 1989 YZ250. Last winter, I rebuilt the top end after reading your article in Dirt Rider. The bore was so worn that I had to skip to a one millimeter-oversize piston kit, just so the bore job would clean up a severely worn spot below the intake port. After I rebuilt the top end, I cycled the engine by letting it idle for three 15-minute sessions with adequate cool-down periods in between. When I first rode the bike, I heard some detonation noises but didn?t think it was a serious problem, until it seized. What could be wrong?
Answer: Your problem is simple. When a cylinder is overbored, the displacement is increased and that boosts the compression ratio. Whenever a cylinder is overbored more than 0.010 inches or 0.25mms the cylinder-head diameter must be enlarged to the new bore size. Otherwise, the piston could contact the head or the edge of the head surface that extends into the bore could cause a hot-spot and pre-ignition. Also, the cylinder head?s squish band must be narrowed by enlarging the combustion-chamber bowl. This also serves to increase the head?s volume, thereby lowering the compression ratio. This work must be performed on a lathe by a qualified tuner or machinist. Average cost of this service is $50
Base Gasket Seeping
Question: I recently rebuilt the top end on my 1991 CR250. I was being as careful as I could be while taking the cylinder off, but the dowels were fused in pretty good and I had to pry it. Needless to say, I gouged the case a bit. I smoothed it out with sandpaper and reassembled the engine. The bike runs great, but a little oil seeps out of the cylinder-to-case mating surface. I assume this is transmission oil? Would it be OK to use something like a thin layer of Permatex Blue or Yamabond here? Would this make it even more difficult to remove the cylinder in the future? Should I just let it alone? The best price I could find on a new left side case was $215 and I?m sure it would be a lot of work and a lot of replacing gaskets along the way. Am I out of luck?
Answer: Air leaks can be very dangerous because the engine could rev independent of the throttle. An inexpensive way to fix your bike?s problem is to draw-file the cylinder base and the crankcases. Then apply a thin coating of Yamabond or any other brand of non-drying sealer to both sides of the base gasket. The best technique for removing cylinders is to tap up on each side of the cylinder with a lead-shot plastic mallet. Remember to put a dab of grease on the cylinder-base dowel pins.
Two-Stroke Exhaust Valves
Three words sum up exhaust valve maintenance: spoogey, gooey, and grungy. If two-stroke exhaust valves didn?t have such a dramatic effect on the engine?s powerband, I?m sure mechanics would remove them and beat them bits with a hammer in frustration because there is little information given by the manufacturers on how to diagnose and repair the exhaust valve systems on well-used dirt bikes. This section is a guide to the characteristic mechanical problems that occur to the exhaust valve systems of dirt bikes. Plus we?ll give you some tips on how to re-time exhaust valve systems.
How Exhaust Valves Work
An exhaust valve system is designed to increase the engine?s low-end and midrange power. There are three different designs of exhaust valve systems. The first-generation design uses a variable-volume chamber mounted to the head pipe to change the tuned length of the head pipe. A butterfly valve is used to separate the surge chamber and the head pipe. At low rpm, the valve is open to allow the pressure waves in the pipe to travel into the surge chamber and effectively lengthen the pipe and reduce the pressure wave?s magnitude when it returns to the exhaust port. These systems were primitive and not very effective on 125cc dirt bikes. Honda and Suzuki used this type of exhaust valve system in the mid to late 1980s.
The second-generation design features valves that control the effective stroke and the time-area of the exhaust port. These valves are fitted to the sub-exhaust ports and the main exhaust port. The main exhaust-port valves operate within close proximity to the piston to control the effective stroke of the engine. The effective stroke is defined as the distance from TDC to when the exhaust port opens. At low rpm, the engine needs a long effective stroke, which results in a high compression ratio. At high rpm, the engine needs a shorter effective stroke, longer exhaust duration, greater time-area, and a lower compression ratio. Yamaha used this system starting in 1982 on the YZ250. Honda?s HPP system is similar and was used on the 1986–91 CR250 and 1990 to current-model CR125.
The third generation of exhaust valve systems attempts to change the exhaust-port velocity, effective stroke, exhaust-gas temperature, and the pressure of the compression wave. Yamaha and Suzuki started using these systems on their 125s in 1995. Both companies employed a venting system to the outside atmosphere. This is very complex because they are attempting to affect the temperature and pressure of the returning compression wave to synchronize it with the piston speed. The exhaust-gas velocity and the effective stroke are controlled by two oval wedge valves that enter the exhaust port at a 45-degree angle. The wedge valves partially block the exhaust port, thereby boosting the gas velocity. Kawasaki?s KIPS system uses wedge valves in the main exhaust port to control the effective stroke, drum valves in the sub-exhaust ports to control the time-area, and a surge chamber to absorb the excess compression-wave pressure at low rpm.
The exhaust valves are opened and closed by a centrifugal governor mechanism. The governor is mounted under the right side cover and is gear-driven by the crankshaft. As the engine rpm increases, the governor spins, thereby increasing the angular momentum of the four steel balls encased in the governor. The steel balls fit into an angled ramp-and-cup arrangement. A spring is used to provide tension on the steel balls. When the momentum of the steel balls overcomes the spring?s tension, and the balls force their way up the angled ramp. A spool attached to the ramp, enabling it to change its linear position with changes in rpm, and the spool is attached to a linkage system that operates the exhaust valves in the cylinder. Factory race teams have different combinations of springs, ramps, and balls to tune the exhaust valve operation and enhance the powerband.
Exhaust Valve Tips and Tuning
Although exhaust valves use the same essential principles, the implementation is different with each manufacturer. Also, each type has its own flaws and fixes. The list below gives you tips on how to install and service the most common exhaust valves, as well as some tuning tips
Yamaha POWERVALVE
Yamaha was the first motorcycle manufacturer to adapt exhaust valves to two-stroke motorcycle engines. Yamaha?s simple design of a cylindrical valve that rotates 1/4 turn to vary the height of the exhaust port requires little maintenance. This system was used on the YZ250 from 1982-98, and on the YZ125 from 1983-93. Occasionally, you have to replace the seals and O-rings to prevent exhaust oil from drooling out of the side if the cylinder. In 1989, Yamaha added a stop plate to limit the travel of the power valve, primarily so mechanics couldn?t install the valve in the wrong position. The stop plate is located on the left side of the cylinder. The valve has a small tab that bumps up against the stop plate to limit the fully open and closed position of the valve. This design enabled Yamaha to position the valve closer to the piston to make it more effective at varying the exhaust-port timing. Unfortunately, the soft-aluminum tab on the valve gets worn, allowing the valve to rotate farther in the fully closed position. Eventually, (after about three years? use) the tab wears enough so the valve strikes the piston, causing damage to the piston. Yamaha?s exhaust valve is cheap to replace. I recommend replacing the valve when the tab wears more than 0.030 inch (0.7mm).
In 1994 Yamaha changed the engine design of the YZ125 and included the next generation of exhaust valves. This system used two oval-shaped wedge valves, positioned at a 45-degree angle over the exhaust port. This system was similar to the one employed by Suzuki. Yamaha experimented with resonator cavity volume, and vents for pressure bleed off and temperature control. Overall this is a very reliable system. Occasionally the pins that fit through the ends of the valve to interface with the actuator lever vibrate out causing the valve to strike the piston. Those pins are a press fit but you can add some Loctite Instant Adhesive to the pins for added protection. One problem that Yamaha is concerned with is high rpm valve flutter. They’ve added springs to the valves to control the flutter but future innovations could include a positive seal between the valve and the cylinders’ valve pocket.
In 1999 Yamaha redesigned the YZ250 engine and exhaust valve system. This model features a powervalve that marks a significant design change, from the company that pioneered the use of exhaust valves on two-stroke engines. Looking more like a Rube Goldberg device, the new powervalve has separate valves for the main (center) and sub-exhaust ports (sides). The whole assembly is controlled by one actuating rod, but the side valves open after the main exhaust valve. The side valves are controlled by two wedge-shaped ramps but you can bet that the factory teams are experimenting with them. The stop plate of the center valve tends to crack, allowing the valve to contact the piston. Look for cracks in the plate richt around the two retaining bolts.
