I just rebuilt one of my 502 Mercruiser engines and after 4 hours of use the #4 piston partially melted. Not sure what caused this and welcome suggestions. The builder installed new aluminium pistons and stainless valves. We have sent the injectors away for inspection and they came back good. All the valves and pistons look clean (even the melted one)like there was no detonation issues. Rings in good shape also. Any ideas what caused this??