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Just did full engine build on my 02 yz250 new crank and top end... broke it in properly and took off pipe and piston has slight score marks in it what could that be from?

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Just did full engine build on my 02 yz250 new crank and top end... broke it in properly and took off pipe and piston has slight score marks in it what could that be from?

Pictures would be a great help.

Also...sometimes a little bit of oil looks like scoring.

Did you inspect and ball hone the cylinder?

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Did you rebuild due to a failure?

You have to make sure the power valve has enough clearance or it will clip the rings. The outside stoppers wear and let it get closer and closer to the piston.

You can add clearance by grinding off metal on the PV edge.(The part that sticks inside the cylinder.)

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I didn't have a choice really my lower crank bearing went out and made the piston chatter around and make scratches in the cylinder

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Your mistake was likely honing a plated Nikasil cylinder,

 

unfortunately you will probably be tearing back into the engine sooner than anticipated

and require once again a new piston, having the cyllinder re-plated or replaced.

Edited by mlatour

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Yep once a nikasil cylinder gets damaged its time for a new one or a replate. Some say you can use a diamond hone but idk.

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Plate it.

Edited by 218

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Really hard to tell from the pictures, but it doesn't look too bad.

 

Send them to Powerseal and see what they say. They may be able to hone it. If not, and the piston doesn't have any deep grooving in it, have them plate to it. Then when you get it back, send the piston to  https://www.line2linecoatings.com/ and have them coat it.

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That looks like that piston found on eBay for cheap. Forget the name of it. Heard a lot of reviews where they explode or start melting

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