Originally Posted by Redcar
i have the same type of pistions 383 60 over with 500 miles on it sound like a break in problem but i just did want the machine shop said what to do but i do have to say that it was running fat for the firts 300 miles
This is classic detonation/preignition damage, the owner even said that he added more timing than what the engine came with. That's most likely what did it in.
Hyper-eutectic pistons are quite intolerant of this as are most cast pistons, but high silicon aluminums including that used in forgings are more brittle and do not transfer heat as quickly as lower silicon alloys.
These alloys also tend to break up, this was mentioned in one of the early replies, if you look carefully at the pictures you can see a crack running from the ring lands into the pin boss. Fortunatly the owner burn't holes through the crowns, lost compression resulting in the engine quiting just in the nick of time leaving him a rebuild instead of a replacement. Pushed a few seconds more, this engine would have been pieces on the pavement.
So when running hyper-eutectic castings or high silicon forgings it is extremely important to stay out of detonation. If you're inclined to push the limits do what the factories do, put a detonation sensor on the engine. It is not difficult to install one such that when it hears pinging it kills the ignition. With a computer in the ignition loop that controls the advance electronically the detonation sensor can be used to back off the advance which is more gentle on the parts than just shutting off the ignition. However, either will save the motor. Detonation sensors don't work with gear driven cams as the gear noise trips them up.