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2003 cr125 top end

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Hey guys I have a few questions. So I just rebuilt my cr125 and I have maybe 10 hrs on it. But I took my exhaust off and looked through the exhaust port and saw a few vertical scratches on my piston. Is that normal? I can't feel them with my finger nail, should I rebuild or is it ok?

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I'd say make sure your rings are seated properly, other than that I'd recommend pulling off the head and cylinder to look for any scoring (scratches) on the cylinder walls.

Another thing is to make sure you warm up your bike good before you get on the throttle hard, lets the metal expand and move better along each other (if that makes sense)

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Some possible causes of scratches: 

 

-improper engine warm up procedure (piston expands faster than the bore)

-cylinder was already damaged before last top end

-wrong sized piston for the bore

-engine runs lean / hot, either from carburetor settings or an air leak

-debris went thru the engine

(past the air filter, parts not thoroughly washed during re-assembly, dirt fell in during rebuild etc.)

 

For peace of mind perhaps remove the cylinder head and have a closer look but

if the damage is superficial / cosmetic and doesn't grab a fingernail, it's likely not serious.

 

Edited by mlatour

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