Thread gauges are used to inspect the screw threads of components and to ensure that they are within the permissible limits. Different gauges are available for different thread profiles like metric, unified, NPT, NPTF, Stub Acme, buttress, BSW, BSF, BA, BSP, BSCY etc. A thread ring gauge is used for checking the diametre and the pitch of threaded shafts whereas a plug gauge is used for the inspection of threaded bores.
The thread gauges are supplied in sets of Go and No-Go. The Go gauge shall engage with the the threaded component without any excessive efforts whereas the No-Go shall not surpass the first three threads of the component. The threading on the component deems acceptable if it satisfies these two conditions.
Some of the common thread profiles are shown below: