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What would cause this type of piston damage??

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Looks to me like it siezed. Overheating causes that. Radiators not cooling enough (clogged or low coolant level?).

While riding your bike, did the motor "lock up" for a while, then free up again, after about 3-5 minutes?

That piston is scrap metal now...

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The rest of the piston looks pretty fresh. The rings have not stuck in their grooves. I'll go with a cold seize......I love these games

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The bike never locked up. It seemed to run pretty damn good, actually. I only took the top end off for something totally unrelated. The piston doesn't really have that many hours on it. It's a Wiseco, btw.

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if that is the intake side maybe it pulled some dirt in at some point. it could have been like that for a while if it was running good before you took it down.

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Yup, it's the intake side. So there's a good chance it sucked dirt?

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Well.....the cut out on the skirt is a bit of a giveaway

Haha, yeah I suppose I could have looked a little closer before asking questions. :smirk:

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How do your reeds and cylinder look?

The reeds look pretty good. The cylinder has slight wear. I had an air leak where the reed cage halves seal together not long ago. It must have sucked dirt in.

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My cr 125's piston kinda looked like that. It was because there was a hairline crack in the cylinder. The streaking look was just smaller than that

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I'm going with a lack of oil that caused a seizure. If you sucked in dirt I would think the top of the piston would have some evidence. The fact that the seizure is over between the transfer ports and the intake ports tells me its lack of lubrication for some reason. (first place that will get starved of oil and overheat)

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That appears to be a lack of lube problem. If it was caused by dirt entering the engine the piston would have a peppered look to it. If it was an air leak or lean seizer it would be on the exhaust side. My guess is that the bike had some very lean fuel or straight fuel ran through it. Maybe Ethanol!

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That appears to be a lack of lube problem. If it was caused by dirt entering the engine the piston would have a peppered look to it. If it was an air leak or lean seizer it would be on the exhaust side. My guess is that the bike had some very lean fuel or straight fuel ran through it. Maybe Ethanol!

X2..... would like to add possible improper cyl to piston clearance (or close to it) and minimal warm up....?

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I've seen engines suck water and score the intake side of the piston like that.

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Looks like a piston I had failed a while age that leaned out with the crank seal behind the magneto went. Check that out too.

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