I had my 502 Mercruiser engine rebuilt over the winter. I also had new plugs, wires and cap put on. After about 4-5 hours of use this summer the motor was done. The original engine builder took it apart and found the #4 aluminum piston melted on the one side with aluminum stuck to the cylinder wall. This motor is fuel injected so we sent the injectors away to see if one or more were running lean but they came back good. Anyone experienced this or what else could it be?