What do you think causes this?


The top of the piston seems ok



Thanks in advance!!!

 

Looks like knock damage to me. More about the engine it came out of? How are the cylinder walls?

 

Knock damage was my first thought, poor fuel quality, too much ignition advance. It looks like combustion flame and pressure damage getting by the top outter edge of the piston. If its not knock related I'd be surprised but its worth considering also that the pistons were not machined to exact tolerances.