There is a gauge you can buy, has multiple blades with teeth of various spacings. Match the blade up to the bolt you have and it will tell you number of threads per inch. I got one of these years ago with a tap and die set. Also you can buy a plate with threaded holes for every size of bolt and thread pitch, metric and SAE.


It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed