That type of crack is usually caused by early timing and/or a lean mixture, which sets up preignition and detonation. It's like hitting the top of the piston repeatedly with a sledgehammer just before the crankshaft reaches the top of the stroke. It can also hammer the bearing material flat on the upper bearing shell.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!