I had what I thought was a reputable place (Dependable Performance Marine aka: House of Boats) rebuild the 455 for me and I think they just stuck a regular automobile motor in my boat with no mods. Its a long story..
Two weeks ago in Havasu, it backfired and quit running with an hour or two on the motor. 25 compression in #1, 0 in #4 and about 150-160 in all others. Oh, #4 electrode was melted out of the spark plug, other plugs looked ok. #1 exhaust rocker nut had backed off and the rocker was 90* from where it was supposed to be (off of the push rod and valve). Very excessive play in the #4 intake and exhaust rockers. Going to pull heads this weekend.. Will pull motor to check bottom end if there is any damage to pistons/cylinders.