Best way to measure rotor bath??

[NoDOHC]

The way that I usually do it is to take a thin piece of lexan or clear acrylic about 4 inches wide and 10 inches long and to drill three holes in it. One in the dead center, and one on each end in the center (2 inches from either edge). Put a film of thick oil on the surface of the rotor (not in the combustion chamber) and pull the plastic down to match the contour of the rotor using a strap around the opposite apex of the rotor (a block of wood across the apex seal groove is handy to avoid cutting the strap). Once the plastic matches the contour, you can fill the dish using the conventional method (syringe or eye dropper).

I have had to make thin metal pieces for the outside to attach the strap to, but usually the lexan bends to the contour of the rotor very nicely.