Thread on fittings: what you need to know.

We regularly receive questions about the threads used on different types of fittings. Is the internal or external thread tapered or parallel? Can different thread types be used together?

Although there are, of course, exceptions to the rule, the following applies to the most commonly used thread types, BSPT (British Standard Pipe Taper) and NPT (National Pipe Taper):

• The different thread types are not interchangeable. An exception is a combination of internal BSPP or BSPG (parallel) thread with external BSPT R (tapered) thread. You might expect BSPT with parallel Rp and Rs threads (which can be confusing when dealing with BSPT threads) to also fit BSPP/G (parallel) threads. However, this is not the case due to differences in nominal diameter.
• BSPT fittings have a parallel internal thread (Rp) and a tapered external thread (R) as standard. NPT fittings have tapered internal and external threads as standard.
• Although the internal and external threads of BSPT and NPT fittings engage with each other and may therefore appear to seal on the thread, the use of a sealant or thread-sealing tape is essential to achieve a leak-tight connection.

BSP or NPT?

Both BSP (“Gas thread”) and NPT are thread standards used on pipes and pipe fittings to connect them together. Neither is inherently better than the other. The choice between BSP and NPT is primarily determined by geographical location. Roughly speaking:

• BSP is mainly used in the UK, Europe, Asia, Australia, New Zealand and South Africa — and in most other countries worldwide.
• NPT is mainly used in the USA and Canada.

BSPT vs. NPT

Both BSPT and NPT use tapered threads, but they differ in the number of threads per inch. The 1/2″ and 3/4″ sizes are an exception, as they have the same number of threads per inch.

However, the threads differ in two other fundamental respects. With NPT, the thread crests and roots are flat, whereas BSP threads have rounded crests and roots. Secondly, the thread angle is 60° for NPT and 55° for BSP.

BSPT on NPT and vice versa?

Although an NPT fitting may appear to fit into a BSPT fitting in some cases (or vice versa), such connections will NOT provide a reliable seal due to the differences described above.

GNS supplies thread adapters that make it possible to connect two otherwise incompatible thread types.

Click here for an overview of the most common thread types, including their abbreviations, meanings and characteristics, or here to email us with your question.

* Taper: gradually narrowing; conical.