Taper Pipe Threads are one of the more challenging threads to produce and measure. The ASME B1.20.1-2013 Pipe Threads, General Purpose (Inch) is the current standard that covers both the part thread specifications and the gages for NPT, NPSC, NPTR, NPSM and NPSL threads. We will be focusing on the National (American) Standard Pipe Taper (NPT) threads. Generally, NPT taper pipe threads are a 60° thread form that is tapered at 0.75 in / ft. (1°47’ half angle). The taper causes the rules for Go and No-Go gage inspection to change as compared to straight threads. To understand the gages, we must first understand how the part threads work and their purpose.

NPT part threads have a tapered internal thread that assembles with a tapered external thread. Per the ASME standard, “The mating threads should always contact on the thread flanks. The design tolerances are such that mating crests and roots may clear, contact, or interfere. This joint may not necessarily seal, unless a sealant is used. ” (See the ASME B1.20.1-2013 for the illustrations of the root and crest clearances.) If you require a pressure tight seal, a sealant must be used on the wrench-tight length (L2 external and L3 internal).