How to properly clay the pistons to check for p-v clearance?

Any kind of clay works so long as it isn't dried out. Take off your cylinder head, so you can see the valve reliefs on the piston. Take a dab of oil (not too much because it can throw off your readings) and rub it where the valve would contact to slick the surface. Then put a small piece of clay there. put the head on and snug the bolts. So long as you know your compressed gasket thickness you should be okay. Make sure to run the engine through atleast two complete revolutions since it is a four stroke engine. I would run it a little more just to be safe. Take the head off, and measure the thinnest part of the clay with a micrometer. You need atleat .1" of clearance to account for expansion and such. Hope that helps.

Yes any kind of Mic will work, so long as you are gentle with the clay. You don't want to mess up your calculations. I personally prefer the digital calipers. They are accurate and easy to use for this; however, that is just what I had on hand, I didn't go out and buy one. You must have put too much oil if the clay kept moving around. You want enough to keep it from sticking, not a thick films so it is hovering on the piston.