The gauge used to check the external thread is called the ring gauge (caliper gauge), and the gauge used to check the internal thread is called the plug gauge. The thread ring gauge and the thread plug gauge are collectively called thread gauges. A thread gauge is a measuring instrument with a standard form of a thread that reflects the boundary conditions of the internal (external) thread being inspected. It is a dimensional gauge. It cannot get the actual value of the thread to be measured, but can only determine whether the thread to be measured is qualified.
To determine the conformity of the threads to be inspected, thread gauges are divided into GO thread gauges and NOGO thread gaugesand are used in pairs. The GO thread gauge can be rotated through the thread to be inspected during measurement, but the NOGO thread gauge cannot be used. If the GO thread gauge can be rotated through the thread to be inspected, but the NOGO thread gauge cannot be rotated through the thread to be inspected, the thread to be inspected is qualified, and the opposite is not qualified. For the plug gauge of internal thread inspection, the "GO" is used to prevent the size of the internal thread from being too small, and the "NOGO" is used to prevent the size of the internal thread from being too large.