Identification of the thread size of unknown bolts or screws is relatively easy provided that it is done in a systematic way and with the correct basic equipment. There are only 2 majors’ screw threads systems that are used for fasteners (bolt &nuts) in the world today and there is a good chance that the bolt that needs identification with falls into one of these 2 categories:
Vernier caliper – for measuring major diameter of bolt
Thread pitch (screw) gage (both inch & metric)
3 Basics steps to follow for bolt thread-identification
*note: the major diameter of a bolt will always measure slightly less than the actual nominal diameter. This is because all bolts have a manufacturing tolerance and for bolts (external thread), this tolerance is on the minus side.