For NPT gauges for U.S.-type pipes, the central notch of the L1 gauge (three-stage notch) indicates the reference diameter, and the other two notches indicate the maximum and minimum allowable notches.
To use the gauge, screw it into the gauge until the screw stops, and if the pipe end of the screw to be inspected is within the range of the maximum and minimum notches, it is judged that the fit inspection has passed.


[How to use NPT gauge (tapered thread gauge for pipe)]

Types of screws	Taper internal thread	Taper external thread
	A: Position of reference diameter	B: Position of reference diameter
gauge	tapered threaded plug gauge L1	tapered thread ring gauge L1
	C: Minimum notch D: Acceptance range E: Maximum notch	F: Minimum notch G: Acceptance range H: Maximum notch