British Standard Pipe Thread (BSP)
The thread on every British tap, boiler and gas fitting.
Origin
Derived from the Whitworth form and standardised for pipework in the late nineteenth century, as gas and water mains spread through British cities.
The catch
A half-inch BSP fitting is not half an inch across. The size refers to the bore of the wrought-iron pipe the thread was designed for, and the thread itself sits on the outside of that pipe, so the actual diameter is closer to three quarters of an inch.
| Nominal size | Actual OD (in) | Threads per inch | Common use |
|---|---|---|---|
| 1/8 in | 0.383 | 28 | Gauge and instrument connections |
| 1/4 in | 0.518 | 19 | Small bore fittings |
| 3/8 in | 0.656 | 19 | Appliance connections |
| 1/2 in | 0.825 | 14 | Domestic water and gas |
| 3/4 in | 1.041 | 14 | Mains supply and radiators |
| 1 in | 1.309 | 11 | Rising mains |
| 2 in | 2.347 | 11 | Industrial and commercial pipework |
BSP comes in two forms: parallel, which seals with a washer or an O-ring, and tapered, which seals by the threads binding as they tighten. Both are still current, still specified in British and European plumbing, and still incompatible with the American NPT thread, which uses a 60-degree angle rather than Whitworth’s 55. Fitting one to the other is a classic way to produce a slow leak that takes a fortnight to find.