How To Identify BSP Threads

BSP Threads

British pipe standard fittings are amongst the most popular threads in the world today.

Here is how to identify a BSP thread.

BSP threads come in two different variants:

– Parallel (BSPP)

– Tapered (BSPT)

The thread flank angle for both tapered and parallel British threads is 55°.

It is fairly common for a Tapered BSP thread to be mistaken for an NPT thread. The way to tell the difference between the two is the flank angle.

The flank angle for NPT is 60° compared to the BSP tapered which is 55° (this can be verified with a thread gauge).

Although BSP is a foreign thread it is not metric.

This is why it comes in imperial measurements such as 1/8, 1/4, 1/2, 3/4 and so on.

BSP parallel threads commonly seal via a 30° chamfer on the male thread to a 30° recessed cone inside the female thread (swivel only).

If it is a port application an o-ring and a washer or bonded washer are needed to achieve a proper seal for parallel threads.

For tapered BSP threads, a seal is acquired via thread wedging with additional support from thread dope or Teflon tape.

As previously mentioned, it is possible for a male BSPT (tapered) to fit into a female BSPP (parallel), so long as the female thread is fixed and not swivel (this is due to the recessed cone seat).

BSP parallel thread size fittings and their specifications can be identified by completing a few calculations.

To find the thread size:

1) Measure the O.D (Outer diameter) of the BSP thread.

2) Take the O.D measurement in inches and subtract 1/4 (.25″).

For example, a BSP parallel male thread measures out to O.D 1″.

Subtracting 1/4 gives a thread size of 3/4 also known as “dash” 12.

Once you have the thread size, you then need to determine the number of threads per inch to verify that it is a BSP fitting.

You can do this by counting the number of thread crests over a 1/4″ length and then multiply this by 4 to get the number of threads per inch.

For example, if you over a distance of 1/4″ you have 3.5 thread crests:

1) Multiply 3.5 by 4 to get 14. This is the number of threads per inch

2) Combine the thread size with the number of threads per inch and refer them to the following chart to find your desired size and its relative specifications.

British Pipe Thread Table


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