I have been reading and doing much preparation work on my latest engine block. I have read a few times about a modification to the coolant (water) passages at the surface of the deck. I am sure that many people have done it. But I want to know what exactly it does or helps with.

David Vizard says to tap and install 1/2" NPT plugs in the passages, then mill them even at the surface. Then drill 3/16" holes in them. Why do this? It seems that it would restrict the flow of coolant. I have heaard it might be to strenthen the deck surface.

 

it's done to strengthen the deck, i think it's 3/4 npt plugs not 1/2

 

Yes it is done to strengthen the deck.

The holes are drilled using the head gasket to be used as a template. To understand why, just go look at a head gasket. Check out the size of the coolant holes. Given the sizes of those, it doesn't matter that the hole in the deck is 1" in diameter or whatever. The idea is, the coolant flow is restricted in places it's not required, so that more flow is available to go through places where it IS needed.... specifically, on the exhaust side, at the center of the bank, to cool the place where there's 2 exhaust ports right next to each other. Note that on many modern head gaskets, there's 1 little hole at each of the other coolant passages, and 3 holes at that spot.

 

Yes, I was wrong. It is 3/4."
Thanks for the explanations.