[HWYSTR455]

Originally Posted by Sirrotica

The 421 mod is drilling a steam hole in the deck surface that the head gasket has a provision hole for, but Pontiac never drilled the hole, except for a few select engines. The 421s were in that group, hence the reference to a 421 modification.

By drilling that hole it is said that it will improve the cooling by eliminating a steam pocket area by venting it to the cylinder heads.

I've never done it either, but probably would, if I were to build a fresh engine from this point forward. It wouldn't cost anything to try it, and eliminating a steam pocket can't hurt anything.

Ah, ok, that makes more sense, and that I too have heard of, and actually have done that on builds in the past.

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