Suzuki rm250 compression

[willt]

Hey Guys, i wonder if anyone can help me. i recently bought a rm 250 1992 year, with low compression. i assumed it would be an easy fix. i have replaced the piston and ring, head gasket and the barrel is in pefect condition. but i can still turn the bike over by hand and the bike wont run. i used a standard 66.9 piston. can anyone help?
:whoa:

 

[RM_guy]

Are you sure it had a std size piston in it? Measure the old one to see if it is near the same size as you just put in. You are looking for a fairly big difference so the location it is measured at isn't that important. The cylinder may have been sleeved and over bored or if it is still plated it might be worn out of spec.

 

[_JOE_]

What was the ring end gap?

 

[willt]

its hard to say because the guy i bought it of, i think already replaced the piston because it looks new. so he's obviously just sold it on. but the piston does feel quite loose in the cylinder. i did measure the bore and it seems to be 70.03 and the piston is 66.9. what do you mean by ring end gap?

 

[THEWELSHM]

Hey Guys, i wonder if anyone can help me. i recently bought a rm 250 1992 year, with low compression. i assumed it would be an easy fix. i have replaced the piston and ring, head gasket and the barrel is in pefect condition. but i can still turn the bike over by hand and the bike wont run. i used a standard 66.9 piston. can anyone help?
:whoa:

Check crank case seal


thewelshm

 

[RM_guy]

...i did measure the bore and it seems to be 70.03 and the piston is 66.9. ...

You have the wrong size piston and rings. The ring gap is the gap between the ends of the ring when it is in the cylinder. It should be .012" or so. With the undersized piston and rings it will be a lot larger than that and won't hold compression.

 

[_JOE_]

Check crank case seal


thewelshm

Crank seals wont cause low compression on the top end.

And ring end gap should always be checked, it's just as important (maybe even more) than measuring the piston to cylinder clearance. I believe you should have .003-.004 per inch of bore.....