The thread standard is the most common detail missing from a ball valve inquiry — and the most expensive one to get wrong. NPT and BSP threads look almost identical, and at some sizes they even screw together for a few turns. Then they leak. Here is what each standard is, where it is used, and how to write it on an order.
The three you will meet most
| Name | Standard | Form | Seals by | Marking |
|---|---|---|---|---|
| NPT | ASME B1.20.1 | 60° thread angle, tapered 1:16 | The threads themselves, with tape or paste | NPT |
| BSPT | ISO 7-1 (formerly BS 21) | 55° thread angle, tapered 1:16 | The threads themselves, with tape or paste | R (male), Rc (female taper), Rp (female parallel) |
| BSPP | ISO 228-1 | 55° thread angle, parallel | A gasket, bonded washer or O-ring — not the thread | G |
Why they are not interchangeable
NPT uses a 60° thread angle; BSP threads use 55°. The pitch is also different at most sizes:
| Nominal size | NPT (threads per inch) | BSP (threads per inch) |
|---|---|---|
| 1/4″ – 3/8″ | 18 | 19 |
| 1/2″ – 3/4″ | 14 | 14 |
| 1″ – 2″ | 11.5 | 11 |
At 1/2″ and 3/4″ the pitch is the same, so an NPT pipe will start into a BSPT valve. The flanks do not match, though, and the joint will weep under pressure no matter how much sealant is used. This is why a mixed-up thread standard often shows up only after installation.
Tapered or parallel?
Tapered threads (NPT, BSPT) seal by wedging: the male and female threads tighten against each other, and the sealant fills the gaps. Parallel threads (BSPP, marked G) do not seal on the thread at all; they need a sealing face and a washer or O-ring.
ISO 7-1 also allows a parallel female thread, Rp, to seal against a tapered male thread R. Many European valve bodies are threaded Rp for this reason. If your market expects Rp, say so — Rc and Rp are both “BSPT” in everyday language, but they are not the same thread.
Other standards on valve datasheets
- DIN 2999 / DIN 259 — older German standards. DIN 2999 corresponds to the ISO 7 taper/parallel pipe thread; DIN 259 to a parallel thread. You still see them on drawings and in purchase lists.
- JIS B0203 — the Japanese taper pipe thread, also based on ISO 7 (R, Rc, Rp).
Our threaded series can be made to NPT (ASME B1.20.1), BSPT (ISO 7-1), BSPP (ISO 228-1), DIN 2999/259, JIS B0203 and BS 21. The standard is fixed at order — it is part of what the approved sample locks in.
Which one does my market use?
As a rule of thumb, NPT dominates in the United States and Canada and in equipment built to American standards. BSP threads — tapered or parallel — are common in Europe, the UK, the Middle East, much of Asia and Australia. But rules of thumb are where mistakes come from. The reliable answer is the thread on the pipe or equipment the valve will go on.
How to write it on an order
- Name the standard, not just the family: “NPT to ASME B1.20.1”, “Rc to ISO 7-1”, “Rp to ISO 7-1” or “G to ISO 228-1”.
- If you are matching an existing valve, measure it: count the threads per inch, check whether the diameter tapers, and use a thread gauge for the angle. Or send us a clear photo next to a ruler.
- Ask for the thread to be marked on the valve body or the box. It saves your warehouse from sorting by eye.
If your purchase list just says “threaded”, send it anyway. We will ask the one or two questions needed to confirm the standard before quoting.