Thread: Decked?

Tweet

You take a light cut of metal off the deck surface of the block (where the heads bolt to the block) in order to set the squish (piston to head clearance).

When the manufacturer built the motor originally, they left the piston down in the bore slightly (maybe 0.025" to 0.075") with the piston at top dead center. Adding the head gasket thickness to this figure leaves the piston too far down in the bore at top dead center (maybe 0.070" to 0.120") to "squish" the mixture across the chamber for proper mixing when the piston comes up to the squish pad on the cylinder head. The tighter you can get the squish, the better the motor will run and the less it will be likely to ping on cheap gas. Most builders will shoot for a squish of 0.035" to 0.045". That means, for instance, if you are using a 0.040" thick head gasket, that you would want the piston crown to be at zero deck, or even with the deck at TDC. That would leave a clearance of 0.040" between the piston crown and the head (it will be less than that because rods stretch and the crank flexes) so that the piston will just barely miss crashing into the head at TDC and will give the mixture a maximum "shot" across the chamber.

To do this properly, you'd use only one piston/rod assembly and you'd assemble it on the crank at the four corners of the motor, one at a time of course. You'd measure the distance from the piston crown to the top of the block deck at TDC and record these figures. I've never yet found a block that was square, they're always off at least a few thousandths. Anyway, lets say that the block is square and that your measurements are 0.035" on all corners. You would then remove the piston/rod assembly, the crank and bearings and take the block to your machinist, telling him how much to "deck" the block so that you had the desired zero deck or wherever you wanted the piston to be at TDC.

If you were going for 0.040" squish and using a 0.040" gasket, you'd tell him to cut the decks 0.035". If you were going for 0.040" squish and using a 0.018" shim gasket, you'd tell him to cut the decks 0.013". That would leave the piston 0.022" down in the hole. Adding the 0.022" to the 0.018" gasket thickness would yield a squish of 0.040"