Plasti-gage is this round compressible plastic that you put between surfaces you want to check the clearance on, torque them to spec, then unbolt and match the now-squished plati-gage width to a scale that comes with the stuff to tell you how much clearance those two parts have. The wider the strip, the tighter the fit. Not as common now as it once was, but some old hot-rodders that couldn't afford precision measuring devices "back in the day" swear by the stuff.