Pat, just need to bring this thread back to life. Have you seen any Mark IV engines that already have the holes on the bottom? I am dealing with a Gen 5 block and a mark IV block. The heads look identical other than port sizes. Even the head gaskets appear the same. I did find one rod in the Mark IV engine that appears to have been bushed and resized. Is this something that could have been done during a reman or do they come factory on occasion? It is a 1990 mark IV engine(truck) and a 1991 gen 5 engine(mercruiser).

This engine is sea water cooled. Would I see any advantage to plugging and drilling the top holes? If so, what size hole do you drill after plugging? I did notice that both engine's head gaskets have small openings on the top row of holes.