I think modeling clay. But that is a different spec. With that you're measuring piston to valve clearance with the cam installed and hooked up.
The piston to head (flat part of the chamber) is what I was worried about with that piston setting so high...
Keith Black says .040" is okay, with .035" minimum. You'll need a thick gasket I'm thinking...
http://www.kb-silvolite.com/test/art...n=read&A_id=35