Two thread systems dominate threaded connections: American NPT and British BSPT. From a distance both look like tapered threads, but they do not fit together.
The differences
| Criterion | NPT | BSPT |
|---|---|---|
| Thread angle | 60° | 55° |
| Crest and root | Flat | Rounded |
| Taper | 1:16 | 1:16 |
| Pitch designation | Threads per inch | Threads per inch, different values |
| Common region | North America, ASME world | Europe, Asia, British standard equipment |
The angle difference means the threads never seat fully. A forced joint may not leak on day one, but vibration and temperature soon start a leak, and the threads are permanently damaged.
How sealing works
In a tapered thread the seal comes from the threads wedging together. PTFE tape or a liquid thread sealant is still used. Tape direction matters: wrap it with the thread, otherwise assembly unwinds it. Too much tape can split the female part.
In parallel thread systems (BSPP, NPSM) sealing comes from a gasket or O-ring rather than the thread. Mixing tapered and parallel threads is another common mistake.
Where it shows up
Instrument connections, gauge and transmitter nipples, drain and vent lines, small bore forged fittings and valve ends. On imported equipment the equipment side is often BSPT while the line is NPT, and an adapter is required.
When ordering
Always state the thread type: NPT, BSPT or parallel (BSPP). Add size, pressure class, material grade and standard (such as ASME B16.11).
See our forged steel fittings page and send your list through the enquiry form.
FAQ
Can NPT and BSPT be assembled together?
They may appear to fit, but the thread angles differ, the joint leaks over time and the threads are damaged.
What is the difference between tapered and parallel threads?
A tapered thread seals on the thread itself, while a parallel thread seals with a gasket or O-ring.
How should PTFE tape be applied?
In the direction of the thread and in a measured amount; reverse wrapping unwinds it and excess tape can split the fitting.
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