BSP (British Standard Pipe) is a family of pipe-thread standards based on the 55° Whitworth thread form and used across the UK, Europe, the Gulf and much of Asia. It comes in two types: BSPP, a parallel thread to ISO 228 (marked G), and BSPT, a tapered thread to ISO 7 (marked R). BSPP is a fastening thread that seals on a washer or seat, while BSPT wedges and seals on the threads themselves. That 55° angle is also what separates BSP from the American NPT standard, which uses a 60° form — the two are not interchangeable.
Most site problems on threaded lines start from one simple misconception: that all BSP threads are the same. In fact there are two different families, and they do not have the same purpose — one is made to seal, the other to hold a mechanical connection. This page explains the difference, which threads TUPY Brazil fittings carry, and how to work out the real size of a part in your hand.
| Feature | BSPT — taper thread | BSPP — parallel thread |
|---|---|---|
| Reference standard | ISO 7-1 | ISO 228-1 |
| Male thread geometry | Taper | Parallel |
| Purpose | Sealing on the thread | Mechanical connection |
| Sealing method | Metal-to-metal engagement with a sealing compound | Separate washer or seat |
| Inspection method | Thread gauge to ISO 7-2 | Go/no-go system to ISO 228-2 |
| Usual designation | R for male, Rc for taper female | G for male and female |
The practical consequence is simple: a part with a parallel thread cannot be made tight just by pulling it up harder. If a joint is leaking and the part has a parallel thread, the problem is not torque — it is the missing washer or seat.
The nominal size of a threaded fitting matches no physical dimension of the part: "half inch" is neither the inside diameter nor the outside diameter. To identify an unmarked part, measure the thread diameter with a caliper and compare it against the table below:
| Nominal size (inch) | Dp (mm) | Db (mm) | C — useful thread length (mm) |
|---|---|---|---|
| ¼ | 12.8 | 11.5 | 9.7 |
| ⅜ | 16.3 | 15.0 | 10.1 |
| ½ | 20.4 | 18.6 | 13.2 |
| ¾ | 25.9 | 24.1 | 14.5 |
| 1 | 32.6 | 30.3 | 16.8 |
| 1¼ | 41.1 | 39.0 | 19.1 |
| 1½ | 47.0 | 44.9 | 19.1 |
| 2 | 58.6 | 56.7 | 23.4 |
| 2½ | 74.1 | 72.2 | 26.7 |
| 3 | 86.6 | 84.9 | 29.8 |
| 4 | 111.4 | 110.1 | 35.8 |
| 5 | 136.7 | 135.1 | 40.1 |
| 6 | 162.0 | 160.9 | 40.1 |
A pressure-tight taper thread is checked with a gauge, not with a caliper. The measuring reference is the "gauge plane" (PC) — the section of the taper thread at which the diameter equals the nominal value.
BSP stands for British Standard Pipe — a family of pipe-thread standards based on the 55° Whitworth thread form, used across the UK, Europe, the Gulf and much of Asia.
A BSPT thread is a taper thread to ISO 7-1 and it seals on the thread engagement itself. A BSPP thread is a parallel thread to ISO 228-1 and is used for a mechanical connection; sealing requires a separate washer or seat.
No. BSP uses a 55° Whitworth form, while American NPT uses a 60° form to ANSI/ASME B1.20.1. They are not interchangeable and must not be mixed on the same joint.
Measure the outside diameter of a male thread, or the inside diameter of a female thread, with a caliper and compare it to a nominal-size table. BSP sizes are named for the old pipe bore, so a ½-inch BSP thread measures about 20.4 mm across the male crests.
Measure the outside diameter of the male thread (Dp) and the inside diameter of the female thread (Db) and compare them against the nominal diameter table; for example a Dp of 20.4 mm and a Db of 18.6 mm is a ½-inch size.
TUPY Brazil BSP fittings have internal parallel pressure-tight threads (Rp) and external taper pressure-tight threads (R), produced to ABNT NBR NM ISO 7-1; socket threads are to ABNT NBR 8133 and ISO 228-1.
Source: TUPY Brazil BSP fittings technical catalogue, pages 6 and 26 to 31.
