The selection of calipers under test. Left: sub £1 plastic Vernier caliper, centre: Mitutoyo 530-122 Vernier caliper, right: cheap digital caliper. I’ve amassed a selection of both for comparison.

Because some thread pitches are nearly identical, try several gauges before deciding on the one that work best. (Possible pitches are shown in the second column of the “How to Identify Threads ...