Thread: Head Gasket coolant passages

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Originally Posted by offshoreonly28

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.

the top holes are closed down on HP blocks with hi boost with blowers some thing not all ways done thats a world block in photo there drill hold is about 5/16 deck needs to be mill after work .the bores may move some at the top of the bores .so if you do this you will want to deck the block and bore block after this work.stock rods from gm i not seen bushed .mercruiser did use a h beam . i think they did tin dip the 7/16 rods .hard to say if the rebuilder did this to save some rod cores i only work on the 375 hp any thing other then this was custom builds with H beam rods .some of the bow tie IV block s have the lower row drilled and some of the stock block s have two drilled .if you used v heads on the IV block you need to drill the holes as well as use the V head gaskets