Head Gasket coolant passages

On Mark IV BBC there are 2 irregularly shaped coolant holes at the rear of the block on each deck, a large one and one about half that size. However every gasket I've been getting the last few years (fel pro) only have an opening for the smaller one. Looking at my old engine building books they show the gaskets with both holes. Why did they do away with gaskets with both holes or do they still make them? If so what is the difference of running the engine using both hole or not?........ just curious :HMMM:

well you have to add some holes or should at the botom of by the lower row of head bolts in the deck BUT the 1017 felpro as well as other still have them. see them 3 holes some bbc have one or two if you use that head gasket you have i have drill out for all 3 holes. felpro put that in as a up date in with some of there head gaskets use your head gasket to lay the 3 holes out i drill them with 1/2 drill

I may be wrong but. I think we're talking about different holes. The two holes near the rear.

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Originally Posted by wrenchaholic

I may be wrong but. I think we're talking about different holes. The two holes near the rear.

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No were not if that hole is plug you want to open the lower row of holes in deck i done about 30+ bbc :D this way it move s the coolant thru the head better they say moving it around the spark plugs less hot spots . i do all the stock blocks this way race blocks are done if you do not want to drill the deck buy a set of head gaskets that are open back there 1965to 90 head gaskets still come with the back holes in the head gasket and i still drill the deck what are you using them on ? race or stock how much CR the Corteco .Rol gaskets and MR gaskets as well as cometics and the felpro numbers F1017-1 .F1017-2 and the big bore stuff F1057. F1093 still have the two back holes and the marine head gasket s that were gray in color felpro/marine had one of the two holes plug in the gasket i am thinking it been some time i have used the old marine gaskets. there is a small paper on telling you to drill the deck of the block .like i said i do about 99 of the stock block big blocks i get i drill them holes

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.

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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.

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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

Thanks for the info Pat.

Have you even seen a 2 bolt Mark IV block with the bottom holes cast in the block? The instructions that came with the head gaskets indicate that Gen 5 and 6 along with some Mark IV truck blocks came with the holes. My block does indeed have them cast in the block.

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Originally Posted by offshoreonly28

Have you even seen a 2 bolt Mark IV block with the bottom holes cast in the block? The instructions that came with the head gaskets indicate that Gen 5 and 6 along with some Mark IV truck blocks came with the holes. My block does indeed have them cast in the block.

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like i said some did only have one and some have had only two . and i drilled so i have all 3 holes. if you use the mark 5 heads with head gasket you want all 3 spots . i have done many i started doing them over 10 years ago mine and customers blocks. felpro tells you to do this .there not casted in the block the lower one () hole is but the rest are 5/8 the last guy could of drilled them or if bowtie block s they have holes in all 3 spots. i never seen alot of the blocks as to where they came from out of car or truck .
if they do not have all 3 spots. i drill them no big deal all race blocks come this way so i do not have to drill them

DO you have any suggestions on keeping the shavings out of a already clean block that I am going to drill out?

Axle grease on your drill bit and blow any remainder out of the freeze plugs with air.

My machine guy stopped by the house the other day and said i gotta drill 4 holes in my block. It's a IV block as well. The holes go between 1-3, 3-5, 1-2, and 2-4 cylinders.