Piston Deck Height

The last time I measured an 850 after a rebuilt -with the head installed -I got 8.2-1.
The valves were sunk a little but not too bad and I did use a .020 base gasket and a standard flame ring head gasket.

I generally fill up 3 threads in the spark plug hole when I measure them.

Barrels and cases do vary a bit. I've seen pistons anywhere from flush to .040 down from the gasket surface.

Mine was 7.9:1 as bought.

But it did have a base gasket. And perhaps the valve seats had been cut.

Like you Jim, I fill to the first 2-3 threads of the plug thread to allow for the shale of the plug when fitted.

comnoz said:

I'm not sure I'm following his numbers but if he only has .050 thou squish distance. That would mean the head has been milled so 9.36 would be believable.

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Jim, I was not factoring in a .050” squish clearance. I was trying to show that’s Glens estimations we’re showing a higher CR than he would actually see in a motor.

Actually Jim I’ve remembered that you and I had a similar discussion a while back. I was concerned I was going to have a higher CR than I wanted when you did my head. But I hadn’t factored in an amount of oil up to the rings, or in the valve pockets, etc and all of that must have added up.

swooshdave said:

I took a more accurate measurement. I get .043". I didn't have a problem with pinging previously and I always use premium. Is it more of a problem at higher elevations?


I have a new composite head gasket and a used copper one. Can you get the copper hot enough with propane or MAP gas to anneal?

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Hi David

Higher elevation eliminates pinging. Not enough oxygen. When I’m in the Sierras above 7,000ft my combat will run on any grade fuel although power is down. I’ve always been worried about putting a thicker base gasket as it moves the cam follower’s normal run in their tunnels. Alternately I glued two copper head gaskets together to compensate for the .040 milled combat head. So far no leaks and no pinging at sea level.