When I used PG to check main bearing clearance in the vehicle, I'd put a small bottle jack under the crankshaft, and top the clearance out so all the slack would be on the bottom side. As long as there's a pretty good metal-to-metal contact with a minimum of oil film between the parts, it's very accurate, compared to an actual measurement of different diameters. For instance, if the plastic string compresses to indicate there's a .002" clearance, that's actually a .001" film of oil all the way around the shaft while the engine is running.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!