ISO 228-155° flankpipe thread
British Standard Pipe (parallel) Threads
The parallel pipe thread marked G. Threads are Whitworth form at 55 degrees, so a G thread does not interchange with NPT even at the same nominal size — 1/2 in the two systems is a different diameter and a different pitch. 11 designations are listed here, all quoted in inches.
Dimensions
in| Designation | TPI | Major (in) | Pitch Ø (in) | Minor ext (in) | Tap drill (in) |
|---|---|---|---|---|---|
| 1/8-28 BSP | 28 | 0.3831 | 0.3602 | 0.3374 | 0.3472 |
| 1/4-19 BSP | 19 | 0.5181 | 0.4843 | 0.4508 | 0.4654 |
| 3/8-19 BSP | 19 | 0.6559 | 0.6224 | 0.5886 | 0.6035 |
| 1/2-14 BSP | 14 | 0.8248 | 0.7791 | 0.7335 | 0.7535 |
| 5/8-14 BSP | 14 | 0.902 | 0.8563 | 0.8106 | 0.8307 |
| 3/4-14 BSP | 14 | 1.0409 | 0.9953 | 0.9496 | 0.9697 |
| 7/8-14 BSP | 14 | 1.189 | 1.1433 | 1.0976 | 1.1177 |
| 1-11 BSP | 11 | 1.3091 | 1.2508 | 1.1925 | 1.2181 |
| 1-1/4-11 BSP | 11 | 1.65 | 1.5917 | 1.5335 | 1.5591 |
| 1-1/2-11 BSP | 11 | 1.8819 | 1.8236 | 1.7657 | 1.7909 |
| 2-11 BSP | 11 | 2.3469 | 2.289 | 2.2307 | 2.2559 |
Sizes are nominal bores, not diameters — 1/8 measures 0.3831 in across the thread. Tapered sizes show a dash for tap drill: a taper is cut with a stepped drill, so there is no single drill size to quote.