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I was wondering if anyone could tell me what caused a big hole in my piston it was a fresh rebuild with just a few hours on it it is a 265cc bore I don’t want to rebuild it and have this happen again any advice would be greatly appreciated
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what was your compression ratio v fuel you were using did you do a uccr! as it also looks like it has been detonating
 

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I was wondering if anyone could tell me what caused a big hole in my piston it was a fresh rebuild with just a few hours on it it is a 265cc bore I don’t want to rebuild it and have this happen again any advice would be greatly appreciated View attachment 64081
I would agree a bad air-leak or extremely low Octane relative to Compression...

Try to determine if the engine overall has no air-leaks first, full leak down test, then perform a complete UCCR & Squish Test. You can look up UCCR & Squish Test that I wrote years ago & posted by BDT Motorsports here on .NET...self explanatory & a real good-safe way to insure you prevent another major piston failure.

Carlos
 

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Discussion Starter #8
I recently replaced the crank seals but On the engine and changed a few water pump pieces but other than that nothing changed that topend came off another bottom end I had ran it on the other motor A little while and piston looked good when I took it apart because bearing in bottom end went out So I changed it to the other bottom end I had and figured I would change crank seals while I was doing I was running 93 octane with klutz r50 mixed 40to 1 could the crank seals have caused that and what is the process for a leak down test thanks for all replies
 

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IMO...you got an Airleak as stated above. You need insure if you do, that you find it, and fix the leak before rebuilding, otherwise you will fry it again.

Carlos
 
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