Bryanj has talked about air gauge measuring of bearings in past posts. I've never seen it or done it myself.....all I have are conventional measuring tools. I imagine it is basically a "leak down" test for bearings?
My exposure to air guaging was for the tooling one past employer made for Federal Mogul spark plug cold forming dies. Federal Mogul bought Champion. The die used to make the hex portion of a spark plug they ram EDM the inner shape then polish it by hand, and the hex portion is polished for size across the flats as well as surface finish.
In that case the air gauge thing was rectangular and it measured that across the flats dimension. No idea the exact clearance they build into the gauging head but super clean metered air leaks around the gauge "member" and how much leaks is measured somehow, today electronically but that older stuff may have used inches of water or gauge oil ??
From a few vids I get the idea that some bearing shells when installed by design do not have a round ID ??
My dad worked for a company after high school that would come out to your house and rebuild the engine in your car at your house, an in frame rebuild is term they use for bigger equipment. He said their standard practice for checking bearing clearances for split bearings was 1/2" square pieces of shim stock that had been cut and had the cut edges carefully peened to remove any burrs. One set of shims went in, caps were torqued and engine should still roll over with the shim between the bearing and the crank, then they took it aprt and put the upper limit shims in, and again torqued, and the thicker shims should lock the engine up. If there was an issue I suppose you could go back and do one at a time. Dad was always a but skeptical about Plastigauge, claimed it could lie, of course they were checking bearing clearances under your car in your driveway or garage :-). I see no problem with using shim stock really but have never tried it. Could be a quick double check method if things were not acting as they measure.
Bill
