ISO 7-155° flankpipe thread
British Standard Pipe (taper) Threads
The tapered pipe thread marked R. Seals on the thread flank like NPT, but at a 55 degree Whitworth form, so the two systems are not interchangeable and their tap drills differ. 9 designations are listed here, all quoted in inches.
Dimensions9 sizes
| Designation | TPI | Major (in) | Pitch Ø (in) | Minor ext (in) | Tap drill (in) |
|---|---|---|---|---|---|
| 1/8-28 BSPT taper | 28 | 0.383 | 0.3602 | 0.3374 | — |
| 1/4-19 BSPT taper | 19 | 0.518 | 0.4843 | 0.4508 | — |
| 3/8-19 BSPT taper | 19 | 0.656 | 0.6224 | 0.5886 | — |
| 1/2-14 BSPT taper | 14 | 0.825 | 0.7791 | 0.7335 | — |
| 3/4-14 BSPT taper | 14 | 1.041 | 0.9953 | 0.9496 | — |
| 1-11 BSPT taper | 11 | 1.309 | 1.2508 | 1.1925 | — |
| 1-1/4-11 BSPT taper | 11 | 1.65 | 1.5917 | 1.5335 | — |
| 1-1/2-11 BSPT taper | 11 | 1.882 | 1.8236 | 1.7657 | — |
| 2-11 BSPT taper | 11 | 2.347 | 2.289 | 2.2307 | — |
Sizes are nominal bores, not diameters — 1/8 measures 0.383 in across the thread. Every size in this table is tapered, so the tap drill column is a dash: a taper is cut with a stepped drill, and there is no single drill size to quote.