A friend of mine has a Marinepower Chevy 350 that blew a head gasket and ruined the block as it cut a passage between two cylinders. I had the sane thing happen with my Marinepower 350, twice. The engines were mildly built, about 320 HP and had the graphite / steel head gaskets. All three lasted about 3-4 years before this happened. Neither of these boats have gate valves. The engines turn at about 4,400 rpms max. I suspected a carb issue with float levels and or jetting issues and or overly advanced timing. He thinks that it is a water pressure issue. Both boats have open cooling systems and are run on rivers and lakes, no salt. Any thoughts? Below are photos. I ran mine for many hours Because I knew it was toast and needed it for a few bass tournaments, so the cracked pistons are probably happened afterwards and are not useful autopsy data.