Andy,
80mm bore
66mm stroke
72cc combustion chamber (this was my '66, what is yours?)
With a 37.4cc dome volume, you end up with 9,4 to 1 with a 1,0mm deck height
To get to 9,8 to 1 compression you need a deck of 0.616mm
For reference, my JE slugs were 37cc dome for 9,5 to 1, but that was with a 1mm overbore to 81.
Here is my thread (from SIX years ago!
) showing my pistons, see how the tops are flat? That is 37cc dome. Do yours look more like a 906, is there a sharp ridge on the top? That is a higher dome than mine. Look at the last photo in the thread-- those are 906 pistons I think, although some say that the 69S pistons were the same.
http://forums.pelicanparts.com/911-engine-rebuilding-forum/368320-jugs-slugs-confessions-je-piston-owner.html