is it time to bore

geewiz

New Member
Mar 12, 2011
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So I removed head today and found stamp on piston (67701) and I can move piston back and forth when piston is at top.the gap between piston and wall is wide enough to stick your fingernail between.So do I need to bore if so how much?
 
motor ran but was smoking badly.the crank seals where bad.I replaced them then did leak down test and found no leaks.I tore It down to measure bore.Did not do compression test.I didn't think the piston should move back and forth that much.
 
i would measure the cyilinder in multiple areas with a bore gauge and inspect the piston. then go from there.
 
there needs to be clearence from the piston to the cylinder wall... it could just be something to do with the rings... so like mentioned you need to measure the cylinder and piston and report back
 
okay I'll have take to a shop.Can I get the cylinder measrued at an automotive shop?The nearest power sport shop is a 40 min drive.
 
just to finish our conversation though........

You said the engine was smoking. Was that from tranny fluid ? If it was leaking from the stator side it would be a lean condition and do just the opposite and not smoke at all and would run at erratic idle speeds.

Then you said you replaced the seals and did a leak down and found no leaks. That would be next to impossible since during a leak down you inject air into the cylinder and depending on how much pressure you injected, some it bound to escape. So a ZERO leak down on a used motor (even a fresh engine) is hard to fathom.

Typically a new piston install would include .0035" - .0045" of piston to wall clearance. So yes, some rocking of the piston is OK and should be expected. A finger nail thickness is more like .040" and thats really wore out. That much of clearance would also show on the cylinder wall and most likely have next to no compression.

Rotate the piston down and inspect the cylinder. Do you see scratches or deep grooves ? With that much clearance I would expect to see damage or some sort.

If the motor ran fine and had reasonable compression and wasnt making "slapping" noises, Id figure out my jetting and run it.

Some things that you have described dont add up to me. Best bet is to take it to someone to look at that has experience with engine machining. Let them determine if the the bore is worn or damaged and needs fixed.
 
when I did leak down I found large leak in intake the psi dropped fast.After sealing intake retested the engine held 7 psi for a good 1/2 hr.Blaster came with a pwk 36mm carb witch is way to big and atv has been running lean.I'm looking for a oko 30mm.
 
when I did leak down I found large leak in intake the psi dropped fast.After sealing intake retested the engine held 7 psi for a good 1/2 hr.Blaster came with a pwk 36mm carb witch is way to big and atv has been running lean.I'm looking for a oko 30mm.

again, if you had an intake leak it would cause a lean run condition and erratic idle, but wouldnt smoke excessively like you mentioned.

I think it could be over carb'd and over jetted and that would/could cause it to SMOKE profusely.
 
36mm pwk with a 158 main/48 low.I think carb is to big and over jetted.Hoping I just need honing and rings.