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Piston hitting case on 96 rm125

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There are marks and you can feel where metal has been ground away on the piston and case....

It is a 1996 rm125 with a 55.5mm piston in it. The bore does not seem to be worn at all...

Anyone have any thoughts as to what might cause this?

125piston.jpg

125case.jpg

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How can the piston even touch the sides at the cases? Doesn't the cylinder have a short skirt that sits into the case when bolted down? Was it running and that happened? Is it a bigger than stock size piston? Maybe the cases were not ground down to fit a bigger bore?

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Good question.... I need to look closer when I get home. maybe it is coincidence that the wear markings are in the same place...

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This bike should have a 54.5 mm piston. The cases should have had those shrouds ground down where the piston it contacting before the oversize was put in.

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so, any oversized piston needs a case mod huh.... I checked and it is the piston hitting the case.

I wasnt sure if this is an indication of bearing/rod wear. But it sounds like you are saying the last guy failed to modify the cases huh.

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Yup. Not all bikes need it, but always make sure what the piston fits without contacting the cases before you install it. You can either grind the cases yourself if you know what you are doing, or have a shop do it. The shops can do it for about $15 and you have the insurance that they won't mess it up.

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You need to take at least 1mm off the cases to fix this problem. Tape up the cases and mark 1mm and get after it. Wash the cases out before you assemble the top end just in case you let some shavings slip by the tape. Yes, you will need the motor out of the frame for this mod.

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