stock 125 compression readings (PSI)?


Jan 31, 2003
I finally did my first top end.
The compression before the top end was 167psi.

After breaking in the top end and riding it under 1/2 throttle for about 20 minutes I get a reading of 180psi.

What range should a stock 125 be?
Bike is a 2001 KX125.

Also, after a rebuild when is the best time to do a compression test? Does it matter if I do one at 20 minutes or will the top end need to be broken in further to get a more accurate reading.

This has been buggin me for a couple days now.
I thought that this bike had a modified head or cylinder (higher compression) but the guy who sold me the bike said he ran pump gas. I do know that the bike was ported, and the company that did it has machined there name into the side
I talked to the company that ported it and they could not tell me anything about the motor, they just knew that they ported it.


Dec 2, 2001
A compression check is a poor way to determine top end condition. Better to check ring end gap. Your compression figures are in the ballpark for a fresh 125. Also, the porting of a cylinder does not necessarily raise static compression.


Jun 3, 2003
That's good advice. I just got a 2003 RM125 (2nd bike). My first one is a '01 YZ250 that I still have. Anyways I've only ridden this RM for 10 minutes(just got it 2 days ago). I took a compression test and it's a an even 200 psi. I've heard it needs to be around 155-195. I love this RM and it feels like the 250 in power.
My question is how long will the ring(s) last? I know you'll probably say 20-30 hours or so. But I ride 90% of the time on a track in high rpm's. And ride once a week. How long will it last. A full year? Half year?

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