SBC deck plugs

[Alan Roehrich]

It's been a while, I think we drilled the small plugs to 5/16", and big plugs to 7/16".

I know 20 years or so ago, when we couldn't do our own shortblock machine work, we had a shop do a block, and they drilled them all 1/4", the thing overheated all the time, and blew head gaskets, but then it was a dirt late model engine. A drag race engine might not over heat or blow head gaskets, but may still have problems, if the holes are too small.

The plugs strengthen the block, and prevent the top of the cylinders from distorting as much when the heads are torqued. If you restrict the flow too much, you create problems. Drilling the plugs until you almost take the Allen socket hex out of them is a good compromise, you still get a good boost in deck strength, and the block still cools good.

When using the gasket as a guide, use a racing gasket, such as a Cometic, or a Fel Pro race gasket, don't use a regular 7733 Fel Pro blue or steel shim stock replacement gasket. The race gaskets normally have smaller holes, and they're sized about right, since they've had feed back on how much flow/restriction is desired.

The other thing to remember is that if you restrict the flow too much, even if you do not have over heating, you could create hot spots, which will cause tuning problems, such as detonation happening even if the timing is not too high and the mixture is not too lean. Coolant flow needs to be balanced.