Is there any reason other than a defective piston that would cause this failure? The piston is from a 1950 Merc bored . 125 " oversize. It is from a low mileage engine operated at usual highway speeds 2500 t0 2700 rpm. Thanks alot for any help you can give me.
Without having the poison in my hands to look at the fracture line hard saying ,but defective is as good as any reason. That IS a strange failure indeed.
looks to me like you have a rod tweaked to the left/right i would check wristpin bushing/ rod bent for wear like that. look at the top ring land,shinney on one side and not the other indicates bent or stress.
I can't say for sure what happened in your case. I can tell you that it isn't unusual for cracks to start from the slots milled into the bottom of the oil ring. That's why race pistons have holes instead of slots. High RPMs put a lot of stress on pistons and rods. I would have expected that was a factor. It appears you might have an old school sand cast piston. If so, that's another factor. Those aren't the best for durability or performance.