“Threaded” is not a complete specification. NPT and BSP threads look similar but do not seal properly with each other, and BSP itself comes in tapered and parallel forms. A threaded fitting order needs the exact thread designation and the mating connection.
The Main Thread Forms
| Thread | Standard | Form | How it seals |
|---|---|---|---|
| NPT | ASME B1.20.1 | Tapered, 60° thread angle | On the thread, with sealant or tape |
| BSPT (R / Rc) | ISO 7-1 / EN 10226-1 | Tapered, 55° thread angle | On the thread, with sealant |
| BSPP (G) | ISO 228-1 | Parallel, 55° thread angle | On a gasket, bonded seal or O-ring – not on the thread |
| NPTF | ASME B1.20.3 | Tapered, 60°, dryseal | Thread interference, with tighter tolerances |
NPT (60°) and BSP (55°) have different thread angles and, for many sizes, different pitches, so they should not be mixed.
What to Put on the Order
- Thread form and standard, for example “NPT to ASME B1.20.1” or “Rc to ISO 7-1”.
- Male or female thread on each end.
- The mating component – pipe, valve or instrument – and its thread.
- Pressure class of the forged fitting (for example Class 3000 to ASME B16.11).
- For parallel threads, the sealing method and seal material.
- Whether thread gauging records are required.
Checks on Receipt
Check thread marking where present, gauge a sample with the correct ring or plug gauges, and confirm that the mating parts in the same line use the same thread form. Mixed threads often only become visible as leaks during pressure testing.
Related: forged fitting pressure classes and butt weld vs socket weld vs threaded fittings.
Frequently Asked Questions
Can an NPT fitting be screwed into a BSP thread?
They should not be mixed. NPT has a 60° thread angle and BSP a 55° angle, and pitches differ for many sizes, so the joint will not seal reliably.
What is the difference between BSPT and BSPP?
BSPT is tapered and seals on the thread with sealant. BSPP is parallel and seals with a gasket, bonded washer or O-ring, not on the thread.
Which standard covers NPT threads?
NPT pipe threads are specified in ASME B1.20.1. Dryseal NPTF threads are covered by ASME B1.20.3.
