how much compression rm125 and when to change top end

[DreamZx93]

Just bought another bike from a guy to use for a practice bike. Its a 01 RM125 with Bills Pipe cylinder, wiseco piston and the head is shaved down 60thou/in. I havent road this thing when it was fresh so i dont know how it should run. Im wondering what the compression should be so i can check it and see if she needs a new top end. Also is there any other way to tell if I need a new top end. Also I have a micrometer can i just pull the head off and measure the bore and all of the jug and piston if so what should they be. As far as i know its all stock bore. Thanks for the help...

 

[sick 96 250]

I think compression should be around 150-165, we just did a top end on my buddies 125 and he was at 105 before the top end and now at 160psi. I may be wrong tho

 

[marcusgunby]

The rms often run a little higher -175-185psi.

 

[RM_guy]

Milling the head will boost the compression too so who knows what it should be. If the piston has a lot of time on it and the compression is still good you may think all is fine but you risk the piston cracking from fatigue. The only way to really know if it needs a new top end is to tear it down and measure everything. You have to have the piston out of the jug to measure it--and that's the part that's going to get the wear.

I always recommend doing a top end when buying a used bike unless you know for a fact when it was last done.