Thread gages( screw-pitch gages) are used to determine the pitch and number of threads per inch of threaded fasteners. They consist of thin leaves whose edges are toothed to correspond to standard thread sections.
To measure the unknown pitch of a thread , compare it with the standard of the screw pitch gage. Hold a gage leaf to the thread being measured, substituting various sizes until you find an exact fit . Look at the fit toward a source of light for best results.
The number of threads per inch is indicated by the numerical value on the blade which is found to fit the unknown threads. Using this value as a basis, correct sizes of nuts, bolts , tap cutters, and die cutters are selected for use.