Short answer: NPT (American, ASME B1.20.1) has a 60° thread angle with flat crests and roots; BSP (British, ISO 7-1 for taper, ISO 228-1 for parallel) has a 55° angle with rounded crests and roots. From 1" to 2" NPT has 11.5 threads per inch against 11 for BSP; from 2-1/2" up it is 8 against 11. At 1/2" and 3/4" both have 14 TPI, so there you have to check the angle or the crest shape. The two do not seal together, and thread tape does not fix it.
1. Why the thread matters more than the shank
A king nipple has two ends doing two different jobs. The serrated shank grips the hose and is sized to the hose ID. The thread end screws into a pump, valve, coupling or pipe, and it has to match whatever is already installed in the buyer's market. Getting the hose size right and the thread wrong produces a fitting that cannot be used, and the problem is usually discovered on site rather than at goods-in.
2. The three differences between NPT and BSP
| NPT | BSPT (taper) | BSPP (parallel) | |
|---|---|---|---|
| Standard | ASME B1.20.1 | ISO 7-1 (EN 10226) | ISO 228-1 |
| Thread angle | 60° | 55° | 55° |
| Crest and root | Flat (truncated) | Rounded | Rounded |
| Taper | 1 in 16 on diameter | 1 in 16 on diameter | None |
| How it seals | On the thread, with sealant | On the thread, with sealant | On a face washer or O-ring, not on the thread |
| Marking | NPT | R (male), Rc (female taper), Rp (female parallel) | G |
Most king nipples are made with a tapered thread, because the thread itself has to seal. A parallel G thread on a hose fitting is normally paired with a sealing face and a bonded washer, which a standard king nipple does not have.
3. Threads per inch by size: where counting works and where it does not
| Size | NPT TPI | BSP TPI | Can TPI alone tell them apart? |
|---|---|---|---|
| 1/2", 3/4" | 14 | 14 | No. Check angle or crest shape. |
| 1", 1-1/4", 1-1/2", 2" | 11.5 | 11 | Only with a gauge. By eye the difference is half a thread per inch. |
| 2-1/2" to 6" | 8 | 11 | Yes. The pitch difference is obvious. |
NPT from ASME B1.20.1, BSP from ISO 7-1. Full pipe OD and NPT engagement length (L2) for every size from 1/2" to 12" are in our king nipple guide.
4. How to identify a king nipple thread in five minutes
- Measure the thread OD with a caliper. This gives the nominal size. A 2" pipe thread measures about 2.375" (60.3 mm) OD in both systems, so the OD tells you the size, not the standard.
- Count the threads per inch. Use a thread pitch gauge, or lay a ruler along the thread and count crests over one inch. 8 TPI on a 2-1/2" or larger fitting means NPT; 11 means BSP.
- If it is 14 TPI or you are unsure, check the angle. A 60° gauge sits flat in an NPT thread and rocks in a BSP thread; a 55° Whitworth gauge does the opposite.
- Look at the crests under a loupe. Flat tops mean NPT. Rounded tops mean BSP.
- Check for taper. Measure the OD at the first full thread and again near the collar. A difference means taper (NPT or BSPT). No difference means a parallel G thread.
5. What happens when the two are mixed
At 1/2" and 3/4" an NPT male will start into a BSP female and turn a few times, because the pitch is the same. The flanks then bear on the wrong angle, the joint feels tight before it is sealed, and it weeps under pressure. From 1" to 2" the half-thread pitch difference makes the joint bind after two or three turns. From 2-1/2" up the thread will not start at all. More tape or more paste does not make a mixed joint safe; it only hides the leak until the line is under load.
The one mixed combination that is allowed by standard is a BSPT male (R) into a BSP parallel female (Rp), which ISO 7-1 permits with sealant. A tapered male into a G port that relies on a face seal is not the same thing and will not seal properly.
6. Which markets use which thread
As a working rule, buyers in the United States and Canada expect NPT. Buyers in Australia, New Zealand, the UK, most of the Middle East and much of Asia expect BSP. Distributors who serve both, or who import pumps from both regions, should keep the two in separate bins with the thread standard on the label, because the fittings are hard to tell apart once they are mixed.
7. How to order the right thread
Write the thread standard next to every size on the enquiry, not once at the top: for example "2 in. king nipple, NPT, zinc-plated carbon steel". If the fitting is going into an existing port, say whether that port is tapered or parallel. Our FDKP (zinc-plated carbon steel) and FDKS (SS304) king nipples are made with NPT or BSP threads on a SCH120 body, from 1/2" to 12".
Frequently asked questions
Are NPT and BSP the same at 1/2 inch?
No. Both have 14 threads per inch at 1/2" and 3/4", but NPT has a 60° thread angle with flat crests while BSP has a 55° angle with rounded crests. They will engage a few turns and then leak.
Can PTFE tape make an NPT fitting seal in a BSP port?
No. Tape fills small gaps on matching threads. It does not correct a different thread angle or pitch, and a mixed joint can hold at low pressure and then fail in service.
Is BSPT the same as an R thread?
Yes. R is the ISO 7-1 designation for a taper male thread, Rc for a taper female and Rp for a parallel female. BSPT is the older name for the same taper thread.
How do I know whether my king nipple is NPT or BSP?
Count the threads per inch. On 2-1/2" and larger, 8 TPI is NPT and 11 TPI is BSP. On smaller sizes, use a 60° or 55° thread gauge, or look for flat (NPT) versus rounded (BSP) crests.
Which thread is used in Canada?
NPT is the normal pipe thread in the United States and Canada. BSP is more common in Australia, the UK, the Middle East and much of Asia.
Need NPT or BSP king nipples?
Sunze makes FDKP zinc-plated carbon steel and FDKS SS304 king nipples with NPT or BSP threads, 1/2" to 12". Send the sizes, thread standard and quantities and we will quote.
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