Plastigauge is used to accuratly measure the bearing clearance when building an engine.

Cut a small strip of Plastigauge the same width as the journal you want to measure, lay it on the journal, bolt down and tighten the bearing cap and then remove the bearing cap again. You get supplied with a card with all the measurements on. Just compare the width of the plastigauge with the little squares on the card until you find one the same and it will tell you what the clearance is.