Originally Posted by Jeff The Ruler
If you take a SBC block to a reputable shop for a street/ strip build and the block is "zero decked", what is the new deck height? 9.025? what is actually done? Is deck height the measurement from the crank journal centerline to the very top ridge of the deck, where the manifold butts up?
OK for an OEM Chevy small block, the nominal measure of a production block from the crank center line to the top of the head deck is 9.025 inches. However, many production blocks are salvaged from manufacturing mistakes and the actual dimension of any OEM unmolested block may be less.
The assumption is that a zero decked block will have the head deck cut .025 inch which will at 9 inches make the head deck the same height as the piston crown. Hopefully a "reputable" shop will measure the true distance first not just bolt the block in a fixture and whack .025 inch off of it.
If you measure 1/2 the stroke, add the center to center length of the connecting rod and the pin center distance to the piston crown it should be 9 inches. Manufacturing tolerance build ups may actually measure a bit longer or shorter. You also need to be sure of the piston pin center to crown distance on replacement pistons, many pistons are made with the assumption that the block will be zero decked, so they're pin center the crown height is lowered .025 inch. If you assume your block is zero decked and you use these "production" rebuild pistons with an .040 inch thick gasket, the piston will be .065 inch from the heads squish/quench deck which will reduce these effects and lower compression below where you think it is. So make double sure of the piston pin to crown height, often called the compression height or distance.
If you're changing rod lengths the compression height also has to change to keep the total of the lengths 1/2 stroke + rod centers + compression height within the height of the block.
Milling the head deck has a similar effect on intake manifold fit as milling the heads, that is all this stuff is lower and closer together, so the intake may require milling of it's base and sides to make an air and oil tight fit. This is especially true when zero decking and head milling both are present.
So this is some stuff to be watching out for and making sure your shop is doing the same for you.