NPTR – Railing Joint Taper Pipe Thread
NPTR is a lesser-known pipe thread standard series defined in ASTM B1.20. This thread is used to connect Railing Joints that require a rigid mechanical connection, such as stair railings.
NPTR External Thread
The external thread of NPTR is the same as the NPT thread except that it is shortened at the small end of the pipe. It yields a larger diameter at the end of the pipe, thus increasing its strength. This shortened thread is noticeable to the professional eye but could be overlooked by a less experienced worker.
- The shortening amount is designated by L6 and listed in the chart below.
- L6 is 3-4 times the pitch.
NPTR Internal Thread
The internal l thread has the exact basic dimensions as the standard NPT thread, with a relief in the fitting’s end. It allows the external pipe thread to “disappear” when fully engaged (For cosmetic consideration).
- The diameter of the relief is designated by Q, and its depth by q.
NPTR: Dimensions Chart
- Dimensions are in Inches
- All the dimensions are the same as in the standard NPT thread chart except for the following:
- L6 – Shortening amount.
- E0 – The pitch diameter at the pipe’s end. (Replaces E1 of the standard NPT thread).
- L2, L5 & L4 should be taken from the above chart for the standard NPT thread and substructed by L6.
| Thread | Pipe OD | TPI | L6 | E6 | Q | q |
|---|---|---|---|---|---|---|
| 1/2-14 NPTR | 0.84 | 14 | 0.214 | 0.7718 | 0.86 | 0.18 |
| 3/4-14 NPTR | 1.05 | 14 | 0.214 | 0.9811 | 1.07 | 0.18 |
| 1-11.5 NPTR | 1.315 | 11.5 | 0.261 | 1.2299 | 1.34 | 0.22 |
| 1 1/4-11.5 NPTR | 1.66 | 11.5 | 0.261 | 1.5734 | 1.68 | 0.26 |
| 1 1/2-11.5 NPTR | 1.9 | 11.5 | 0.261 | 1.8124 | 1.92 | 0.26 |
| 2-11.5 NPTR | 2.375 | 11.5 | 0.261 | 2.2853 | 2.4 | 0.26 |
| 2 1/2-8 NPTR | 2.875 | 8 | 0.5 | 2.7508 | 2.9 | 0.38 |
| 3-8 NPTR | 3.5 | 8 | 0.5 | 3.3719 | 3.53 | 0.38 |
| 3 1/2-8 NPTR | 4 | 8 | 0.5 | 3.8688 | 4.04 | 0.38 |
| 4-8 NPTR | 4.5 | 8 | 0.5 | 4.3656 | 4.54 | 0.38 |
