Compression test on an rm125 03

[flight]

I have an rm125-03

I have bought all the parts to do a top end rebuild!! But i don't ride all that hard. I raly take it out of third w/ full throttle.

I just bought a compression tester for it but the book dosn't say anything about what the compression should be. I did a good break in of the engine. it has about 15-17 hours on it.

When do the test. On a new set of rings what should the compression be? I would like to ride it one more time but i am aware of the terminal damage that can be caused if i don't change it before rideing.

 

[marcusgunby]

I would honestly go more on hours used than compession readings-i knew of a KTM that broke the skirt of the piston and wrecked the engine, it had a comp reading of 160psi 20 minutes before the piston let go.If you dont ride hard and clean the airfilter regular, and use a good premix at a sensible ratio i would say 25 hours on a piston is ok.

 

[Glitch]

Save yourself money and do what marcus said.  You can always tear down the top end and check the cylinder wall and piston for wear with a micrometer, using the wear limit in you manual as a guide as to when to replace stuff.  If the cylinder wall and piston check out, then just hone the cylinder and replace the rings and do a slight break in to let the rings seat themselves.

 

[flight]

ty for the input but at the moment it easier said then done being that I don't have a ball hone or a micrometer. now that i have my gear tools are my next prority.

Does anyone know a good site for tools i need to get a ball hone and a micrometer?

 

[marcusgunby]

I will be honest and admit i dont generally hone my cylinders at piston change-i might be in the wrong on this one, but so far i havnt seen any need.

 

[Hon-Yam-Junk]

Originally posted by marcusgunby
... broke the skirt of the piston and wrecked the engine, it had a comp reading of 160psi 20 minutes before the piston let go.

I would agree 160psi is too low for compression and piston is worn out. Not to mention pistons made in Japan are low quality Aluminum. Japan does not have the resources to mine thier own Aluminum, but to buy it from other Third World countries.

 

[darringer]

I don't ball hone the cylinder either. If there is decent cross hatch I just use a scotch-brite pad in a criss-cross pattern to clean up the cylinder. Then I give it a soap and hot water bath until a white paper towel rubbed in the cylinder comes out spotless.

 

[Rcannon]

I too have used the scotch brite and never had a problem. i did have a local machine shop hone the cylinder of my YZ just because they offered to.

The thing that helped me was the post a while back that said to wash the cylinder after honing. There was a shocking amount of crap that washed out.