is my top end blown?

[BlasterMaster15]

i was riding yesterday and grabbed the clutch and the rpms started dropping as i slowed down until it died and wouldnt start back up. it has realllllly low compression and i only got it started by pulling it with another blaster. i didnt ride it but my friend03blasterider said it was making a scratching noise when it revved higher. ive never replaced the top end since i got it. would this be a blown top end?

 

[sicivicdude]

More than likely, yes.

Pull starting or "bump starting" will usually start an engine with low enough compression it can't be kick started. It's not advisable except as a measure to get your butt back out of the woods as it can lead to a broken piston which could result in a lot more damage than simply a top end rebuild.

You need to do a compression test (to determine the extent of the damage) and a leak down test to determine if any other repairs are needed. Once those have been done, assuming the compression test is low but the leakdown test comes back negative, you can take the top end off and see how bad it is.

Measure the cylinder to get the current bore size, taper, and out of round. Once you know how far up the cylinder needs to be bored, you can order the piston you need and have a machine shop bore the cylinder to match the piston.

 

[BlasterMaster15]

should i bore it first or get the piston first?

 

[acezeesawtooth]

you may luck out and only need a cylinder hone and a set of rings. that would be the best scenario. on the other hand you may need to get the cylinder bored out. depending on how mechanically equipped you are you make just want to take that engine to a shop to have them work on it. unless you feel comfortable doing it yourself

 

[sicivicdude]

^^^ x2 but if the engine is to the point it has to be bump started, it probably can't be honed and re-ringed but I like the optimism!

You should measure your top end at it's widest (biggest around) point and purchase a piston one size larger than that. For example, say you have a stock 66.00mm bore but it's been all jacked up. Most of the cylinder (the areas that didn't get the aluminum and steel meeting) will still be about 66mm but the areas that gouged could be up to 66.17mm at it's widest point (just an example of the size obviously) because of the scraping that happened. So you would want to order a 66.25mm (.010" or 1st oversize) piston and take the cylinder to a machine shop. They should bore the cylinder to just under the piston clearance size and then finish the bore by power honing to the exact clearance stated. For wiseco that's .0022", most cast pistons are .0015"

If your cylinder is mostly 66.00mm but at it's widest point is 66.27mm, obviously you can't stick a 66.25mm piston in there... you'll need to order a 66.50mm piston and get the cylinder bored to 66.50mm

Most machine shops will, for a fee, measure your current bore size (at it's widest point) and order a piston for you. They will charge you retail for the piston, to bore the cylinder, and a diagnostic fee for the measuring.

If you are not mechnically inclined, you should probably do the compression and leakdown tests to ensure your top end is blown but your crank seals are not and take the cylinder off and simply drop it off at a machine shop. It'll cost a little more but it will all be done all at one time.

 

[BlasterMaster15]

where do you get the leakdown tester?

 

[sicivicdude]

Holeshotman (member here) sells them for $30 or $35 shipped. The parts normally run about that from a hardware store but you can also build your own if you feel froggy.

You need a low pressure gauge, a schrader valve, an expansion plug for the exhaust port, and plumbing to hook the gauge and schrader valve upto the intake boot. They're not complicated to build or use but you need to test your engine before you take it apart to find out if you had an airleak and that's why your top end went.

It could have just been time for it to go, but if it's an airleak and you get the cylinder bored and reinstall without diagnosing the airleak you'll just be back on here in a few weeks asking why it happened again...

 

[99LRDblaster]

should i bore it first or get the piston first?

You get both at the same time.

 

[hudack]

id send it to ken for the bore hone and he will order you the size pistion you need

 

[sicivicdude]

The problem with mail ordering a bore and hone is the shipping costs. It'll cost you $75 to get a bore and hone by the time you pay UPS to carry it both ways. Plus this is Ken's busy season so he's pretty backed up now anyway...

 

[99LRDblaster]

The problem with mail ordering a bore and hone is the shipping costs. It'll cost you $75 to get a bore and hone by the time you pay UPS to carry it both ways. Plus this is Ken's busy season so he's pretty backed up now anyway...

Exactly. Look for a local machine shop that does cylinder boring. You need to know what size you are at currently, so it's a wise idea to check the piston and mic the cylinder to know where you are at. If it's a stock piston, the you need to bore it to the next oversize piston you are purchasing which can be 0.010 or 0.020 over piston.

 

[StrokedTater244]

you need to have the bore measured before anything. it could be out of round badly and need bored .010 .020 .040?? you will not know until it is measured. then you can order a piston and have the shop bore the cylinder to match the piston.