Why is my deck height 2mm and what can I do to reduce it?

[Walt Fricke]

I think confusion can arise over the relationship between clearances, squish zones, and compression ratios.

You want to know deck height (distance piston circumference is below cylinder top) as part of one of the various methods of computing CR. Most of them involve computing head volume, piston dome volume, and some require adding deck height volume (a slice like a coin) to head volume.

If you actually have flat head pistons (and no valve reliefs in them), all you need is dome volume and deck height, but that doesn't apply to any of us Porsche guys.

I just saw an ingenious method of calculating CR, though it is probably only worth it for a shop like Ed Mayo's. He measures head volume in the usual way - plug in head, plastic cover over where cylinder goes, drip the fluid from the burette in to measure. Pretty standard.

But he also had a machine shop make a head shape out of clear plastic, with a hole in the top to let the fluid in. He knows exactly this fixture's volume. So he can run the piston up to TDC, put on the plastic head, hold it down with some nuts on the studs, and fill it. No need to measure deck height doing it this way, as that all comes out in the wash.

I've taken to just setting up one cylinder and head with the motor on a stand with a spark plug hole pointing straight up, running the piston up to TDC, and filling through a spark plug hole. I made a fixture so I can measure how far above the plug my fluid is when I stop, and I can measure or compute that as a subtraction. While at the end of the filling I can rotate the engine some this way and that to convince myself I don't have air bubbles. My results (not many) have been pretty consistent in the amount of fluid dripped in, and I see I was far from the first guy to try it this way - one measurement is all it takes, since you know the swept volume from using math on the bore and stroke. But most seem to prefer more traditional methods, with plastic plates with holes in the middle.

Deck height generally is going to give you a clearance which is usually a sort of floor for actual clearance between piston and head. If the piston dome shape and the head concave shape are the same radius, this could be the same everywhere. And actual clearance could only be greater, not smaller.

But with anything but a stock build, it is prudent to measure all clearances just in case. Especially valve clearances.