auto vs marine engine

block wise there's nothing really other than the freeze plugs need to be brass and some high-end builders will do that on automotive applicates as well. Ring gaps are differnt and piston material is different but again some builders will use the correct pistons auto or marine as well.

Alternator, starter, distributor, and carb are marine spec, carb has flood protection, electrical has spark protection.

then you have reverse rotation engine which are all marine specific.

the wall clearance on the pistons is .0015 to .0025 and usually they go on the high side because of piston scuff, but i have used car engines in the past for marine

For what it's worth, long ago in the seventies I worked for a GM gray iron foundry that poured engine blocks and heads. I can remember changing a line over to run a few "marine heads". I have no idea what the difference was?

I am trying to remember what was different about the heads. The only thing I recall is that the same green sand patterns were used between the autos and marine but the marine heads used different cores. This would mean that the internal passages would be different but the outside of the castings would be the same. They were poured with the same grey iron as the auto heads?