Deck height is the distance from the top of the piston at TDC to the block deck. Most piston/rod assemblies will come up short, lowering compression ratio, and causing(very small) flow inefficiencies. Zero deck means machining down the block to flush with the pistons. Most people leave 0.020".
I don't agree that most people leave it at .020. Zero deck to me means to deck as close to zero as possible. My current small block is set at .001 deck.
There are two operations, cutting to true the block making both surfaces the same distance from the crank "parallel", decking for a specific finished distance from the piston to the deck surface, referred to as "zero decking".
Ryan was right on the money. Zero piston deck is zero piston deck, meaning zero clearance between the top of the piston and deck of the block. As in... zeeero. "Decking the block" or "squaring" the block and then milling it to a specific tolerance is part of the blueprinting process, but is definately not zeroing (is that a word?). There will always be some piston rock back and forth in the cylinder, so zero deck may never always be zero. (never always?) So, what Wally says about getting as close to zero is correct. Arias pistons and some of the TRW's that I've used, rate their BB Chevy piston compression ratio's based on a .020 deck height with a given head chamber volume. If you want the exact compression ratio that the piston manufacturer gives, then you try to get as close to that deck height as possible (We have yet to put one at .020). In a lot of the off-the-shelf pistons I've used, .020 deck height is the number that is used. Zero piston decking is especially beneficial on an engine with "quench" area above the piston. Like engines with closed chamber heads.
[This message has been edited by Glenn Kent (edited 06-12-99).]
If you know something we don't, then throw it out there DZ. Just because people disagree, doesn't mean it's personal. We're all trying to help. You can always E-Mail if you think it's innapropriate to do it here.
I would like to add that most of the books I have read (yes I can read) say that having the correct spacing from the piston to the cylinder head, measured in the "squish" area, will actually reduce the tendency towards detonation. Once again, this is on a closed chamber type head, where there is a flat area that comes together with a flat area on the piston. Apparently when this happens, the fuel / air mixture is squished out of this area and towards the spark plug side of the head, helping combustion by causing turbulance in the combustion chamber. Depending on who the author is, the figure is somewhere around .040". With "0" deck and a .038 compressed gasket, you get very close to this figure. So you could have a 9 to 1 motor with .060 squish and have more detonation than a 9.5 to 1 motor with .040 squish distance.
Most of the late 80's 5.0 Ford motors came from the factory with the pistons sticking out of the block around .005" and you rarely see one with any detonation problems, even with massive doses of laughing gas.
Are we having fun yet
Advanced Automotive Machine
[This message has been edited by BillK (edited 06-12-99).]
Yes, the zero deck height also applies to BBC's. In a max. effort engine, The engine builders will actually take a mold of the combustion chamber. Then they make allowances for Piston-to-valve and Piston-to-head clearances. This way the piston takes up as much of the combustion space as possible. This helps achieve the high compression ratios needed in such engines (15 and 16:1).
The down side of this is in a BBC open chamber motor, the domes get pretty big and can hinder flame travel across the top of the dome (ie:mountain).
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