Taper pipe thread dimensions with gauging diameters at hand-tight and wrench-tight engagement.
NPT is a taper pipe thread, which is why its dimensions look nothing like a machine screw thread: the gauging plane sits L1 in from the small end and the diameter grows at 1:16 along the thread. The tables give the pitch diameter at both hand-tight and wrench-tight engagement, because a taper thread gauge is set to those planes and not to a single diameter.
Basic dimensions to ASME B1.20.1, adopted in India as IS 554. Thread angle 60°.
| Pipe sizein | TPI | Pitchmm | Pipe ODmm | Pitch Ø at L1in | Pitch Ø at L2in | L1in | L2in |
|---|---|---|---|---|---|---|---|
| 1/16 | 27 | 0.941 | 7.95 | 0.28118 | 0.2875 | 0.16 | 0.2611 |
| 1/8 | 27 | 0.941 | 10.287 | 0.3736 | 0.38 | 0.1615 | 0.2639 |
| 1/4 | 18 | 1.411 | 13.716 | 0.49163 | 0.5025 | 0.2278 | 0.4018 |
| 3/8 | 18 | 1.411 | 17.145 | 0.62701 | 0.6375 | 0.24 | 0.4078 |
| 1/2 | 14 | 1.814 | 21.336 | 0.77843 | 0.79178 | 0.32 | 0.5337 |
| 3/4 | 14 | 1.814 | 26.67 | 0.98887 | 1.00178 | 0.339 | 0.5457 |
| 1 | 11.5 | 2.209 | 33.401 | 1.23863 | 1.25631 | 0.4 | 0.6828 |
| 1-1/4 | 11.5 | 2.209 | 42.164 | 1.58338 | 1.60131 | 0.42 | 0.7068 |
| 1-1/2 | 11.5 | 2.209 | 48.26 | 1.82234 | 1.84131 | 0.42 | 0.7235 |
| 2 | 11.5 | 2.209 | 60.325 | 2.29627 | 2.3163 | 0.436 | 0.7565 |
| 2-1/2 | 8 | 3.175 | 73.025 | 2.76216 | 2.79063 | 0.682 | 1.1375 |
| 3 | 8 | 3.175 | 88.9 | 3.3885 | 3.41563 | 0.766 | 1.2 |
| 4 | 8 | 3.175 | 114.3 | 4.38713 | 4.41563 | 0.844 | 1.3 |
| 6 | 8 | 3.175 | 168.275 | 6.50597 | 6.54063 | 0.958 | 1.5125 |
Internal threads are checked with a thread plug gauge — a matched GO / NO-GO pair — and external threads with a thread ring gauge or a calibrated snap gauge. Setting plugs keep the rings in calibration between recertifications.
DSN manufactures NPT plug gauges, ring gauges and setting plugs at the Coimbatore works, cut and lapped as matched pairs and issued with traceable calibration certificates. Send a drawing or a thread designation and we will quote to it.