2000 rm 125 compression

Just installed new piston and ring and my compression is still low. What else will cause low compression?

Worn cylinder most likely, outside chance head o-rings are blown. Did you hone the cylinder for a better ring seal?

I took the cylinder to the place I bought my piston from and he said the cylinder looked good. So did no work to cylinder

How much compression? Looks good must mean he can tell the difference in .003 to .005 (by eye) because that is the difference between barely ok and needing re-plated.

Compression is reading 70 after new ring and piston. Before it was reading 58

It should be around 150 to 170psi after holding throttle wide open and kicking at least 10 times.

I had it wide open with fuel off. Ok I'll try it again later this evenning.

So being a rookie at the self mechanic situation I bought a pro xpiston kit @53.95mm when the last piston was a wiseco 54.00mm. So I will be installing the wiseco in the am hopefully that does the trick

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