Head and piston question!!

Hey guys I’m new on here and have a few questions for you. I bought an rm 125 I’m rebuilding. The guy said he rebuilt the engine but the compression was at 90. I popped the head off and found these things I’m questioning. The head looks like there is some pitting on it, and super light pitting on the piston. The ‘pitting on the piston is so light I can feel it. The cylinder looks good and there is no scoring or nicks. It fire right up and runs smooth but doesn’t have the compression I’m looking for. Do I need to get the head refinished? Should I replace the piston and rings? What should I do ?

Any input would be helpful. I was going to buy a new piston but it looks brand new and has very little play, it barely moves around in the cylinder, but does move slightly. The head is what worries me.
 

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SRAD97750

Moderator
Staff member
Have you tried another compression tester? I like to compare with two set ups, because when one seal in the tester fails, it will read lower than it should.

You'll need to measure your piston ring end gap clearance to tell what needs replacing.

Everything looks really good from the pictures. I wouldn't worry about the prior detonation damage to the head, it all looks pretty smooth.
-BIG DAN:thumb:
 
Have you tried another compression tester? I like to compare with two set ups, because when one seal in the tester fails, it will read lower than it should.

You'll need to measure your piston ring end gap clearance to tell what needs replacing.

Everything looks really good from the pictures. I wouldn't worry about the prior detonation damage to the head, it all looks pretty smooth.
-BIG DAN:thumb:


Thanks for the reply dan! I’ll give another tester a shot. You don’t think the head pitting can cause a compression issue? Any idea on what that clip is called?
 
Have you tried another compression tester? I like to compare with two set ups, because when one seal in the tester fails, it will read lower than it should.

You'll need to measure your piston ring end gap clearance to tell what needs replacing.

Everything looks really good from the pictures. I wouldn't worry about the prior detonation damage to the head, it all looks pretty smooth.
-BIG DAN:thumb:


Dan thank you for the tester idea, bought a new one and it’s showing 160psi
 
honestly, sorry for the bad news but looks like he blew it up, threw a new piston in it and put it on craigslist. head needs to be cleaned up, cylinder sent out and you need a new piston. see that black spot in the piston? thats an aluminum burn from either the cylinder shavings or the remnant of piston shrapnel from the old piston. you should be able to find a good head on ebay for $30,
 
If the crank isn’t sloppy spend the $300 and do a new top end. And get a new head that one looks like crap! But first try and stick a magnet to the inside of the cyl. If it sticks, throw it in the garbage. The cylinder not the magnet. The magnet is still useful :prof:
 
If the crank isn’t sloppy spend the $300 and do a new top end. And get a new head that one looks like crap! But first try and stick a magnet to the inside of the cyl. If it sticks, throw it in the garbage. The cylinder not the magnet. The magnet is still useful :prof:


Powerseal USA can plate steel cylinders.
If it were me it would get a crank too.
 
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