You can use your dial indicator, and place it on the side of the retainer, but you will need to have the ability to put light tension checking springs on the cylinders you are checking. If you dont have a degree wheel, you will have to check the distance by rotating the crank in slight increments at and around TDC in overlap. The valve basically becomes an extension of your dial indicator, and you would zero the dial before starting, and push the valve further open each time you rotated the crank, noting if the valve appears to get closer to the piston or farther away from it, trying to locate the spot that it is the closest to the piston. An older method, is to place modeling clay on the piston tops, and then rotate the crank over several complete revolutions, and then remove the head. Of course, you will need to install all the pistons and adjust the valves for zero lash when doing this, and you can omit the head gasket, and just lightly snug the head down with a couple of head bolts. Once you remove the head after rotating the crank over, you can take an exacto knife and cut thru the clay to see how thick the impression is from the valves touching it. The problem with this method is that if you are real close with your P/V clearance, you could inadvertantly cause them to hit the piston if you rotate it over too fast before you realize it. You would also add to the thickness of the clay, the gasket thickness and valve lash if its a mechanical lift cam. The degree wheel method lets you sneak up on it by letting you see if you are getting too close before you have contact, plus you can see in thousandths, how much clearance you have.
