You measure deck height from the crankshaft centerline, to do this accurately the crank has to be removed and the distance from the crank journals to the deck needs to be measured with a height gauge on a surface plate or a large micrometer with a ball end adapter.
Of course no one does it this way unless you are squaring the block up, most just check the depth of the piston from the top of the deck and go from there. With this method you are assuming the block is square and the crank centerline is straight and parallel to the deck as well as the length of the rods are identical, pin heights are the same etc.
When you check the distance the piston is down from the deck just remember there are running clearances to account for in your measurement, crank clearance, piston pin clearance, piston rock at TDC etc. Make sure you check all the bores since the stroke can be out a few thou from throw to throw....piston pin heights vary too.
In other words don't get carried away getting the pistons "zero" decked when the engine is static, everything grows at rpm and all those clearances stack up. Also that deck is a different shape at running temperature than the bore since it is a plate of cast iron and you don't want the rings riding in that area by having the pistons coming up above deck height at rpm.
Be conservative, you can always run a thinner head gasket to get compression or more squish in the chamber.
As for pushrod length, it depends on if you machine the deck a lot or not, if all you take off the deck is a cleanup cut and your running a regular hydraulic cam there is a tolerance built into the hydraulic lifters that will allow no changes to your valve train. Do a lot of machining on the deck and realign the crank bore and things can get out of line.
Thats why you check it during mockup for proper geometry of the valve train. For the most part though a typical cleanup of the block and touch up on the crank centerline with a hone with a normal hydraulic cam and you won't need special adjustable pushrods.
It really depends on what you do and how much you do to it and what you intend to do. In other words,,,we need more info to make a recommendation.
Last edited by 4 Jaw Chuck; 11-29-2008 at 12:36 AM.