[Jac Mac]

Detonation! Now as to its reason for occuring it could be one of several causes. Its a little odd in that is adjacent to the inlet valve pocket which would be a cooler part of the chamber. My guess at this point would be stale fuel or possibly a dud injector on that cyl, detonation or even pre-ignition might have caused the head to lift slightly and then coolant pressure has pushed the fire ring of the gasket into the chamber. Is this the motor you have been using for hi-speed runs etc? a lean out at hi RPM would cause this. Also when you pull the piston check the ends of the top ring for signs of 'butting' contact to make sure you have enough ring gap, these motors run the top ring quite close to the top of the piston and in your particular case/usage it may require more initial gap to prevent this and the localised overheating that can occur.