How do I measure a small piston to head clearance

[tubman]

Another way to do this is to use small (pea-sized) balls of aluminum file stuck to the pistons and valves with a dab of grease. I just did this to my engine last week an it worked slick. I just picked up my heads today from the machine shop after being milled .025 on one side and .010 on the other. I now have .045 over 5 pistons, and .043, .047, and .055 on the other 3, which I am satisfied with. (I don't want to be grinding on my heads either.) These measurements were obtained with the heads snugged down over the old gaskets that were in excellent shape.

I had aluminum foil at home, but no Play-Dough, so I went that way. I had no problems with little balls slipping or changing shape when I removed them from the surfaces. I was always was kind of worried about a clay-type medium stretching or deforming when I removed it.