Wonder how this happened (ebz8001)

[Michael G]

Too much compression?

 

[hedge hog]

Heat?


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[Conquistador3]

Yes. Cracked piston crown in an Otto Cycle engine = too much compression or (more rarely) over advanced ignition.
Thermal fatigue is also a possibility but that usually happens on Diesel Cycle engines and it's exceedingly rare in Otto Cycle engines.

 

[LegDeLimber]

Was it running when the piston cracked?
Has anyone ever used a "piston stop" on it, at any time?

Hard to tell from the photo, is the dome thin?

 

[Michael G]

Not sure. Someone threw this ebz8001 at me for free.

Replaced the piston and carb and starts in a pull. Runs great.