BSP vs NPT Threaded Ball Valves: How to Tell Them Apart
Short answer: BSP (British Standard Pipe) threads use a 55° thread angle and are the norm in the UK, Europe and much of Asia. NPT (National Pipe Taper) threads use a 60° thread angle and are the US standard. The two are not compatible: even where the threads per inch match, the different thread form means the joint will not seal reliably. Order the valve with the thread that matches your pipework.
BSP threads: BSPP and BSPT
BSP is based on the Whitworth thread form, with a 55° flank angle and rounded crests and roots. It comes in two versions:
- BSPP (parallel, "G" thread, ISO 228-1): a parallel thread that does not seal on the thread itself. The seal is made by a bonded washer, O-ring or face seal, or by sealant on a parallel-to-taper joint.
- BSPT (taper, "R/Rp/Rc" thread, ISO 7-1 / EN 10226): a tapered thread (1 in 16 on diameter) that seals on the thread flanks with PTFE tape or a thread sealant.
Most screwed ball valves sold in the UK have female BSPP ends and are installed on BSPT male pipe threads with a suitable sealant, which is a standard and reliable combination.
NPT threads
NPT (ASME B1.20.1) is a tapered thread, also 1 in 16, but with a 60° flank angle and flat crests and roots. It seals on the threads with tape or compound. NPT is used on equipment built to US standards, on oil and gas skids and on a lot of imported instrumentation.
BSP vs NPT: comparison
| BSP (BSPP / BSPT) | NPT |
|---|
| Thread angle | 55° | 60° |
| Crest and root | Rounded | Flat |
| Taper | BSPP parallel; BSPT 1:16 | 1:16 |
| Standards | ISO 228-1 (parallel), ISO 7-1 / EN 10226 (taper) | ASME B1.20.1 |
| Where used | UK, Europe, Asia, Australia | USA, Canada, US-specified plant |
Threads per inch
| Size | BSP TPI | NPT TPI |
|---|
| 1/4" | 19 | 18 |
| 3/8" | 19 | 18 |
| 1/2" | 14 | 14 |
| 3/4" | 14 | 14 |
| 1" to 2" | 11 | 11.5 |
At 1/2" and 3/4" the pitch is the same, which is why BSP and NPT parts will sometimes start to engage. The thread angle is still different, so the flanks do not seal and the joint may weep or loosen under vibration. Do not mix them.
How to identify an unknown thread
- Measure the outside diameter of the male thread (or inside of the female) with a vernier.
- Check whether the thread is parallel or tapered by comparing the diameter at the first and last full threads.
- Count the threads per inch with a thread gauge.
- If the TPI is 11 or 11.5 or 18 or 19, that settles it. If it is 14, use a 55°/60° thread gauge to check the angle.
BSP and NPT ball valves from Ball Valve UK
See all screwed BSP and NPT ball valves.
Frequently asked questions
Can I use an adaptor between BSP and NPT?
Yes. Male BSP x female NPT (and the reverse) adaptors are made for exactly this. That is the correct way to join the two systems, not forcing one thread into the other.
Do BSPP ball valves need PTFE tape?
When a female BSPP valve is screwed onto a BSPT taper male thread, the seal is made on the taper, so use PTFE tape or an approved thread sealant suitable for the media. Where a parallel male is used, the seal must come from a washer or O-ring.
What does "G" mean on a ball valve?
G is the ISO 228 designation for a BSP parallel thread, so a G1/2 valve has 1/2" BSPP ends.
Still unsure? Send us a photo and measurements and we will identify the thread for you.
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