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Why did my piston break?

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I just rebuilt the top end on my Cr125. The next day i break it in the way the manual says. Warm up ride for 10 mins no more than 1/2 throttle... Finally i actually get to ride it.. I take it for a ride out at this pit in the middle of no where which was a bad idea. I was out there for maybe 1 minute until I hear boggggggggggg. good thing I always carry a rope in a back pack everywhere I go. So i was towed all the way back and took my bike apart and a pretty big chunk of the piston was missing on the top near the ring locating pin is. What could of caused this?

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Ring not centered on the pin, stuck out and caught a port? a picture would help everyone ID the problem easier.

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i wish I took one before I gave my cylinder, piston, and head to a friend to bring to the shop. The ring was still in place all the way around . The cylinder wasn't damaged either. Just a few marks on the walls of the piston.

i just want to see what others think before I hear what the shop has to say even though i cant explain too much without a picture.

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well with out a picture we cant help you, you say it bogged ? maybe a lean condition and you melted the piston idk with out a picture all we can do is speculate

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Ok im bored with out my bike so I just decided to see what you would think maybe Ill have a pic by Tuesday.

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forgot about the pic before I brought it today. If there are a bunch of little pits in the dome, could that have broken the piston. And s studs for the head weren't tightened all the way. Those were really the only things wrong just a bunch of little pits in the dome part of the head. Also the dome was flush and even with the rest of the head. Im running pump gas, the guy at the shop said that's no good. The cylinder didn't get to damaged but still needs work. Im also getting the cases split while the bikes there. How long does that take for a bike shop? 2 hours? So new piston, new head, split the cases, good thing Motosport is having an oem sale, Ill probably get the head while its 25% off and probably the piston.

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pits on the piston top = detonation / spark knock. 125's aren't compression monsters 125's don't have high compression ratios or a high compression PSI reading, should be OK with 93 octane on a stock engine. Were you running 87 octane regular gas ? Detonation will bust a piston if it is sever enough and is let go on for an extended period of time in that condition.

Edited by Atieri

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Regular as in 87 octane ? did the motor make metallic noises ? like marbles bouncing around in the front of your exhaust pipe ?

Always use 93 octane and up in a 2 stroke.

I was running regular gas

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I don't know if its 87 octane. I go to the station, i get 4 gallons of regular. I didn't really notice any noises, before the rebuild it was running fine with regular.

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When I get the head and bring it to the shop do I just tell them Im gonna run premium? Will they have to mill it for premium or what?

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