Wayne,
Right on, those are good, appropriate values for piston-to-valve clearances.
I didn’t give specific numbers because they depend on application and the degree of effort one is willing to go to in order to get “close.” There is lots of argument about how close to run these parts. Every builder seems to have their own numbers. There is no “pat” answer, only “safe” (too much clearance) answers. Too close is when there are signs of impending contact. Not close enough leaves the squish area looking the same carbon pattern as the combustion chamber and can leave room for detonation. Someplace just shy of ever any contact is the right clearance.
Look at:
http://forums.pelicanparts.com/showthread.php?threadid=148258
emcom5’s stock SC piston shows proper squish on the left, rdane’s piston shows reasonable squish around the perimeter and the “shadow” of the intake valve. To the contrary is Bill Verberg’s RSR piston that shows consistent “gun metal gray” coloration over the entire crown. I interpret this image that it had no proper squish (too much clearance) and was detonating. On the image “98mm Mahle & JE” note the nice clean ring around the perimeter of the used piston. That is good squish.
I have seen many (too many) pistons embossed with the contour of the valves – not good for the machinery. That should only happen above 9000 RPM or jumped cam timing. Many (most?) times when you see broken rings and major failure around the perimeter of the piston, it is from detonation in the squish area. Overheated and seized pistons are the other concerns. This may be a “chicken or egg” situation.
Above all: NO CONTACT, just close.
Best,
Grady