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YZ125 Squeal of death

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Hey everybody, I'm relatively new to the dirt bike scene and have a question for y'all.  So I did an engine rebuild on my 2002 YZ125, crank, piston, etc. Started up fine but when I went to break it in I got about 10 feet before it started squealing like a pig. Im really kinda stumped here and before I go to tear back into it wanted to know what to look for/any ideas y'all might have, thanks.  

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Previous to this rebuild, had the engine experienced a failure?

Was the new piston size properly matched to the cylinder ?

During re-assembly, were all clearances checked as per the service manual specs. ?

 

Start troubleshooting by slowly turning the engine over by hand to feel for resistance/rough spots,

remove the exhaust pipe and have a peak at the piston/cylinder thru the exhaust port.

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There are (were?) many threads in this forum where YZ125s & 250s make a scary squeal or screech, and the culprit was a dry bearing due to a low oil level in the transmission. If that is what you are experiencing, just ensure that your gearbox has the recommended 660cm of oil. Drain, measure what comes out (for reference), and re-fill with the correct amount. Often this will silence the noisy bearing with lubrication and that will be the end of it.

Do NOT use the Check Screw to determine the correct oil level; it will leave your bike without sufficient trans oil in the gearbox.

Good luck, and keep us updated with what you find. :thumbsup:

 

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