Originally Posted by Frosty1956
recently bought a 1980 corvette with the original 350 but has had the block decked and owner did not know how much was done. How can I tell I want to add after market heads and intake. He did state the springs on the stock heads had been shimmed
Well Duh!!!!!!! Yea the its been milled off but the other codes are period correct for a Vette. Since material was removed you would assume that the shims were to get the proper clearance to keep the valves from striking the top of the pistons. I mean I know its been a lot of years since I did a motor but I dont think Ive forgotten everything. He had misplaced the receipts where he had the motor work done. Had a new crank, cam, rods and pistons put in it when it was done and what sounds and feels to be a pretty hefty cam. Im just looking at aftermarket heads and intake and was wondering if anyone could tell me how to measure how much had been removed when it was decked. That much I,ve forgotten..............
OK, The term Zero Decked relates to how high the block's head deck is above the piston crown in the case of a 350 with a 3.48 inch stroke divided by 2, plus a 5.7 inch long rod center to center of the bearing to pin bore and the compression height of the piston which standard is 1.56 inch from the pin bore center to the top edge of the piston crown. This should be 9 inches. The production standard block from the center-line of the crankshaft to the head deck is 9.025 inches. Zero decking is removing some to all of the .025 inch making the deck surface even with the piston crown edge. Not all factory blocks have all of this clearance either, Blocks and other parts are sometimes damaged in the manufacturing process and is not so badly damaged to the point of being scrapped, they are salvaged with extra boring or milling as is required. So sometimes you find a factory zero decked block or an over-sized bore, sleeved lifter bores etc. However, a factory zero deck block will have the stamped codes where a block re-machined in service would not.
Zero decking would not affect valve springs, however, a valve job that cut new seats into the head and onto the valve makes the valve stem tip sit higher (sinks the valve into the seat and the seat into the pocket) which lengthens the spring's installed height which reduces its installed pressure, to restore this a shim is often used under the spring to return it to the nominal compressed length when the valve is closed. What zero decking does is to change the distance between the cam and lifter to the rocker arm to less than designed which is the pushrod lenght. Though this usually isn't so much of a distance change that the pushrod length needs adjusting to restore proper rocker arm geometry between it and the valve stem. But it's always good practice to check.
In terms of telling how much of the deck has been removed, there would be no way of telling without at least pulling a head and even then it isn't a guarantee.The reason is that there are rebuilder pistons out there that from the pin center to the crown edge are .020 to .025 inch shorter than the the 1.56 OEM production down to 1.54 inch. This restores the factory clearance to the deck and with that the factory compression with the OEM head. So you can find a zero decked block that measures the factory piston to deck clearance because of this.
The downside of your block being decked is that even though the casting codes may be period correct, there is no way to prove that this is a numbers matching situation where the engine is native with the chassis rather than just loosely related to it. The lack of receipts is not a good thing both in that regard and in the fact that you have to take what's been done on faith. Getting more power out of a 1980 350 whether a 'vette or a pick up is hardly difficult, those engines were so anemic from the factory that the simple act of boring and milling with the other standard machine shop restorative procedures will substantially improve their power output by accident of renovation of the dimensions giving more displacement and compression. Add to that a little cam and it feels like you've bought a real race car.