head work
- Thread starter blastblast
- Start date
Thank you very much for your generous offer, but my son "Triplecrown" and I have extra heads including a couple that suffered Foreign Object Damage that will make great candidates for modification. Now that he has used the modified head I am pretty sure he will never put a stock head back on a running Blaster. Our limitation is time. Both of us work and I am heavily involved with Ground Search and Rescue, as well as the time we spend out in the woods biking, canoeing or camping now that summer is here. I also have other dirt bikes and street bikes and a car project or two on the go, as well as taking or giving an after hours course here or there. I don't even own a television any more!
My son has spotlessly cleaned out the garage (which always happens just before a project, never after!) and is planning a port and head job on an air cooled engine. The present air cooled engine was built for low rpm torque (dropped cylinder, stock ports, head) to (successfully) test the theory. The goal is one step at a time to a high rpm 32+hp air cooled engine that mimics the DT200 engine he is presently driving, but with more low end torque than the DT200 makes. It will rev to 9500rpm but our hope is that it will be stronger than a stock Blaster from idle to that rpm, even at the lower rpm.
The mods will be, one at a time:
1) raised exhaust port and intake port, the modified head will shaved (hand filed and checked on glass!) to keep quench distance right.
2) Epoxy fill of parts of the transfer ports and reed box to keep velocity up and increase crankcase compression
3) toroidal chamber head, giving up some upper rpm power to gain in midrange
Stock carb and either the stock pipe or an FMF or test something else after step one. Sticking with the stock carb because it is a known quantity, to leave it out as a variable. He is using one on his DT200 motor so we know it will make the power. Step one should get us to the max rpm power, steps 2 & 3 are more about mid range.
I'll try to get some pictures, including pictures of the modified and DT200 heads. If you search post by "best" or "triplecrown" there is more info, pictures and video here on this forum.
Blaster thru deep water
I understand the time constraints, it would have been a nice addition to the threads that's been on the subject the last little bit.
So, "triplecrown" and I had a day of it.
His DT200 Blaster had an "undetermined incident" on his last outing and barely made it home under its own greatly reduced power. 13 minutes of work had the motor out and sitting on the floor, draining coolant, radiator, and all. Turns out it was an exhaust side skirt broke off the piston. Rats! Maximum revs and running hot took its toll. Odd that it would be the exhaust side, it is the intake side that seems weak. Still he showed me another piston from another motor that showed a crack on the exhaust side. I guess if you are going to rev these motors hard you had better run a forged piston.
Well, moving on with plan "B".
We ported the intake and exhaust on an air cooled cylinder to match the DT200 specs.
The exhaust port was raised about 0.100" as was the intake to match the DT200. See:
DT200-Blaster-KTM250-power-comparison
The head was shaved another 0.010" with a hand file to test for detonation and motor put together again with sealer only (cylinder 0.040" low) mainly because we did not have a basegasket on hand, but we will say it was to do a fair test of the porting. Square edges on transfer port edges at bottom of cylinder were rounded off just because they were so nasty but nothing else touched with the grinder. No porting, no rounding of the intake, no polishing of the exhaust, partially because we were on a schedule to get this running before dark. Just raised intake and exhaust ports 0.100" and chamfered the sharp edges off.
Before the porting, this engine with the lowered cylinder was a tractor motor. Rock steady idle, pulled away in 3rd gear, tons of mid range torque, much more than a stock Blaster but no revs. This changed the nature of the engine. Starts a bit harder, does not idle as smoothly, but now she revs! Not as peaky as the DT200 (ports still 0.040" lower) Mid range is less than it was but still strong, stronger than the stock Blaster by far. Throttle response is crisp, jetting may need more work but seemed fine.
With the higher revs it gave my 250SX powered Blaster a good run for my money on top speed and as always, I love the gearing on these Blasters. I don't think it is making as much power as his DT200, Nor is it as peaky just yet, but raising the cylinder may change both of these.
Triplecrown has some pictures, head comparisons, DT piston damage, and the work done. I hope they turn out and he posts them. We will run this for a while, get a feel for it, check for detonation, and plan on raising the cylinder soon. I think we may do some epoxy work filling in the intake reed box and some problems in the transfer ports before that. Gonna do some ridin' first!
His DT200 Blaster had an "undetermined incident" on his last outing and barely made it home under its own greatly reduced power. 13 minutes of work had the motor out and sitting on the floor, draining coolant, radiator, and all. Turns out it was an exhaust side skirt broke off the piston. Rats! Maximum revs and running hot took its toll. Odd that it would be the exhaust side, it is the intake side that seems weak. Still he showed me another piston from another motor that showed a crack on the exhaust side. I guess if you are going to rev these motors hard you had better run a forged piston.
Well, moving on with plan "B".
We ported the intake and exhaust on an air cooled cylinder to match the DT200 specs.
The exhaust port was raised about 0.100" as was the intake to match the DT200. See:
DT200-Blaster-KTM250-power-comparison
The head was shaved another 0.010" with a hand file to test for detonation and motor put together again with sealer only (cylinder 0.040" low) mainly because we did not have a basegasket on hand, but we will say it was to do a fair test of the porting. Square edges on transfer port edges at bottom of cylinder were rounded off just because they were so nasty but nothing else touched with the grinder. No porting, no rounding of the intake, no polishing of the exhaust, partially because we were on a schedule to get this running before dark. Just raised intake and exhaust ports 0.100" and chamfered the sharp edges off.
Before the porting, this engine with the lowered cylinder was a tractor motor. Rock steady idle, pulled away in 3rd gear, tons of mid range torque, much more than a stock Blaster but no revs. This changed the nature of the engine. Starts a bit harder, does not idle as smoothly, but now she revs! Not as peaky as the DT200 (ports still 0.040" lower) Mid range is less than it was but still strong, stronger than the stock Blaster by far. Throttle response is crisp, jetting may need more work but seemed fine.
With the higher revs it gave my 250SX powered Blaster a good run for my money on top speed and as always, I love the gearing on these Blasters. I don't think it is making as much power as his DT200, Nor is it as peaky just yet, but raising the cylinder may change both of these.
Triplecrown has some pictures, head comparisons, DT piston damage, and the work done. I hope they turn out and he posts them. We will run this for a while, get a feel for it, check for detonation, and plan on raising the cylinder soon. I think we may do some epoxy work filling in the intake reed box and some problems in the transfer ports before that. Gonna do some ridin' first!
Yes, well, in our case just putting a base gasket back in there.
We are running with just Yamabond sealer on the cylinder base joint at the moment.
This lowers the cylinder by the thickness of the gasket, which is about 1mm or 0.040" thick I believe.
Raising or lowering the cylinder raises or lowers the rpm range the engine works at. We did it to find out by how much.
Steve
I have started reading the 2 stroke tuners guide but haven't gotten to port timing yet. I didn't realize that a base gasket was that thick. So you raised the ports by .1" (2.5mm) and now putting the base gasket back in you will be lowering them .040" (1mm) which will make the over all change of .060" (1.5mm). In the Ken O'Connor video's he showed raised it 1mm and kept the original ark of the OEM port. Are you changing the shape any? Not knowing anything about it but it would seem like the people doing the multi ports would be getting better life out of the engines because they can get the area or the openings bigger and support the rings better. Have you had any experience with adding ports instead of just enlarging them?
Base gaskets are between .025" and .028" squished. Brands (and even individual one between brands) are different. So we're talking about .55mm-.6mm
Yeah, I was running on memory on that one, you are probably right Sicivicdude. 0.025" to 0.028"
I ran out to the garage to look for the basegasket on the broken DT200 motor but Neil had reassembled it to keep it together and from losing pieces.
Good move but not helping me at the moment.
I found the basegaskets that came with the Vito's stroker crank, 0.020" and the plate is 0.039". 0.025" to 0.028" sounds right.
My KTM comes with select fit gaskets so you can set the piston flush with the top of the cylinder or fiddle with your quench distance. Neil and I have done the same with his Blaster engines over time using basegaskets cut from paper and cardboard. What we found is that if we lowered the stock Blaster head (by dropping the cylinder) very much, we got detonation.
dksix said:
I stand corrected on the basegasket thickness, glad you guys are paying attention!
The goal was to just raise the ports 1-1.5mm but "Oops!" 2.5mm was where it is at when I came around to do the first measurement! No problem, we'll run with it. This is the same height as the DT200 so no great damage done. Let this be a warning to anyone new with a grinder, the metal comes off fast, it doesn't go back on very easily!
We (Neil and I) squared up the top of the port a bit, less arc than stock and with a bit smaller radii in the corners. The arc is needed to save the rings from snagging so you flatten it at your own risk. With a flatter top to the port there is a sharper signal pulse to the pipe and generally a sharper powerband. The smaller radii make the top line (slight arc) of the port longer without widening the sides. Do not touch the sides. Nor did we touch the bottom or anywhere on the "funnel" or volute of the port out to the pipe.
The port was like a "D" laying on its flat side before, it is now more of a flattened oval. Flat top and bottom.
So, the total movement of the exhaust port over stock is 0.028" down plus 0.100" up at the moment for 0.072" or about 1.85mm. The head is closer now too. Neil nor I checked the quench with solder (race to get it running!) but it should be in the 0.060" range now.
I have never cut side exhaust ports into a cylinder, but all my KTM engines have them, and it sounds like a good direction to go. The KTM engines open and close them in addition to the powervalve. I would not widen the exhaust port or you will have ring or ring pin problems. Nor would I narrow the intake bridge, since it is the same width as the piston bridge so nothing is gained. The piston skirts are weak (we have had 6 or more fail) so you don't want to take metal off of them.
Two-Stroke Tuner's Handbook
Is the BOMB for info and guidance on tuning up these motors. Written in 1973 it is still valid and hardly surpassed as a good place to start reading today. Jenning's theories and formulas work, it is worth the effort to try to understand them.
Today I am going to do a drawing and cut a template out for the machine shop on Monday. Going for a toroidal shape to fit on this engine with a basegasket under the cylinder. I want the chamber fairly shallow, but I do want to get rid of the remnants of the cone chamber that recesses the plug. May have to make some compromises (deepen the chamber, shave a lot off the face, leave some cove). Any suggestions?
heres a picture of our modded head
Sweet! Excellent photo. I know you have more photos, stop holding out on us!
I can see the headgasket sealing marks so this is from last Saturday just after disassembly, before we filed it down again.
This head as it is has what, about 10-20 hours on it? It came apart this clean, no sign of carbon.
You can see the remains of the cone shaped Blaster chamber. We (it is Neil's engine) debated cutting it deeper to remove this but there are important reasons to keep the combustion chamber shallow.
You can also see the criss-cross file pattern on the head sealing surface. By using a coarse file ("hand bastard" is the proper term!) you can remove a lot of metal and still keep the surface flat. You stroke the file evenly across the head in one direction putting pressure on the top of the file with one hand, taking care not to rock the file (or it will round the edges). Then after you have a pattern going one direction, you file at 90 degrees so you can see where your low and high spots are. You have to measure too so you don't take all the material off one side of the chamber. Neil checked it with sandpaper on glass but I believe my filing was flat enough for use. This surface finish is plenty good for a head gasket surface, and is better than a glass smooth surface. It has some "tooth" to it, holds gasket and sealant better. I did use a slight smear of Yamabond on the much reused copper head gasket.
Incidentally, the head stays nice and clean like that. I don't really know why?
My KTM heads do too. No carbon build up in any of my 2 strokes I have pulled apart in the past 10 years.
Some of the parts engines we have bought and my old snowmobiles have been terribly carboned up. I blame it on better oils and 40:1/50:1 ratios.
Any other opinions?
And my new head, also finely polished:
I didn't think it was possible to add any more power on pump gas but I believe I actually did. I installed the head last night and was tearing up grass with it immediately. I was sliding around the side of the end of the yard and pulling a wheelie coming out of the turn sliding. I did about 4 one wheel wheelies in a row before deciding that I was going to kill myself if I didn't stop!
I didn't think it was possible to add any more power on pump gas but I believe I actually did. I installed the head last night and was tearing up grass with it immediately. I was sliding around the side of the end of the yard and pulling a wheelie coming out of the turn sliding. I did about 4 one wheel wheelies in a row before deciding that I was going to kill myself if I didn't stop!
Sweet!
Is that head cut to your drawing? (which I cannot seem to find at the moment)
And what are you running for squish and CC's?
How did Twostroker99's head work? Notice a flatter cut (no bore step), must be a stock stroke or shimmed cylinder?
What is he running for squish and CC's?
So many questions...
Looks like Triplecrown's head is less squish area and shallower but sounds like results are the same. Rippin'!
I think less squish gives less mid-range and more over-rev, but hard to prove without a back-to-back comparison head.
To all who want to just copy what they see here, I want to make clear right here that we are experimenters, not experts.
For the same price any machine shop will charge to cut your head, Ken O'Conner or other experts will give you their years of experience.
There are no shortcuts. We are not giving away any big secrets, just sharing our experiments.
One question at a time man!
Both twostroker99's and my head were designed using MSV calculating software. The ability of my hands to match the software's output is the limiting factor at the moment (I could do better with a radius tool than by "X'ing and Y'ing"a rounded cutter!) but i'm reasonably close to reproducing what the program calculates.
Twostroker99's head is cut for a stock stroke engine. He's got a trail ported engine, stock (66mm since 1999!) bore stock stroke blaster with a jetted VM26, Vforce3 reeds, and a FMF SST pipe. His head squish is set at .050" with a theoretical volume of 19cc's which is was able to hit fairly close for a given cranking pressure of 142 psi. Improvement over stock (by leaps and bounds) but no where NEAR out of the safe region. He's worried about longevity more than ultimate POWA so I stayed well on the safe side while improving power delivery and fuel efficiency.
My head is cut for a 72.5mm piston on a 3mm stroker crank. I'm running a F7 "top end" pipe so I cut my head to .055" (wider for the higher RPM range!) with a theoretical chamber volume of 17cc's. I rode it around the yard for approximately 20 minutes the other night but have yet to measuring the cranking pressure (it's supposed to be right at 165psi
). I'll update when I measure it and let you know!Are you rechambering your head using hand tools or a lathe? It seems like the transition from squish area to combustion chamber is a little "weak". That looks like it would result in quite a bit of spill in that area. Obviously you don't want it razor sharp because it would be a hot spot, but too turned down and the speed in that area drops off before the charge is "blown" inwards towards the spark plug. Everything I've read (and experimented with) shows about 1mm being the rounding preferred.
Yes, you can go anywhere between no radius and about 3mm but the "sweet spot" seems to be between 1mm and 1.5mm for maintaining charge speed but not having a hot spot there.
heres a video of me riding with our head mod. jetting wasnt bang on in this video it was running rich.
Blaster - YouTube
jumping it in 2nd but it launchs better in 3rd. sprockets are 14/40 i want to go to a 15 in front
Blaster - YouTube
jumping it in 2nd but it launchs better in 3rd. sprockets are 14/40 i want to go to a 15 in front
