I have been getting conflicting input about whether my 75 2.7L engine uses a head gasket (sealing ring) or not - the two experts I asked said yes ("there's a groove in top of the cylinder that a sealing ring sits in..."), Wayne's 911 engine rebuilding book says no in one section and yes in another, the internet generally says no, and when I disassembled the engine I didn't any sealing rings or other evidence of a head gasket between the cylinders and the heads.
A head, piston, and cylinder from my engine:
So I decided to do a little engineering mock up / experiment. I set a head upside down on my workbench and then took one of my new cylinders (which do not have a groove in the machined surface that mates to the machined surface on the head) and placed it upside down onto the head mating the two machined surfaces. They fit together precisely and in fact there was a noticeable suction when I pulled the cylinder away from the head.
I have concluded that my engine does not use a sealing ring between the cylinders and heads.
Another subject I have been undecided about is how to measure deck height. Wayne's book says if you have flat top pistons then measure the distance from the top of the piston @ TDC to the machined sealing surface of the cylinder. But if the piston is domed then use the hollowed out piece of solder technique from Walt at Competition Engineering.
Again, I performed an engineering mock up / experiment. With the head still sitting upside down on my workbench, set a piston top side down onto the head inside the annular machined sealing surface to see what part of the piston contacted the combustion area of the head.
It turns out the 2mm wide flat annular ring on the OD of my 9.5:1 JE pistons contact the beveled annular ring adjacent to the machined sealing surface on the head. That fact leads me to conclude measuring the deck height directly between the piston and the cylinder will be satisfactory.