How to ID British (BSP) Threads

British Pipe Standard fittings are amongst the most popular of all foreign threads in the world today‭. ‬These threads come in two‭ ‬versions‭: ‬parallel‭ (‬BSPP‭), ‬and tapered‭ (‬BSPT‭). ‬The thread flank angle for both tapered and parallel British threads is 55°‭. ‬NOTE‭: ‬It is a fairly common mistake to identify a BSPT‭ (‬tapered‭) ‬as an NPT‭. ‬Always remember that NPT threads have a 60°‭ ‬thread flank angle‭, ‬and BSPT has a 55°‭ ‬angle‭ (‬this can be verified with a thread gauge‭). ‬Although BSP is a foreign thread‭, ‬it isn’t actually metric‭. ‬This is why it comes in imperial sizes‭: ‬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 washer or a 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 thread into a‭ ‬female BSPP‭ (‬parallel‭), ‬so long as the female thread is fixed and not swivel‭ (‬this is because of the recessed cone seat‭).‬

BSP parallel fittings and their specifications can be identified by a completing 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 ‬inch‭ (.‬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, ‬then multiply it by 4‭ ‬to get the number of threads/inch‭. ‬For example‭, ‬if over a 1/4”‭ ‬distance you find 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 to the following chart to find your desired size and its related specifications‭.‬

To identify exactly which fitting you have‭, ‬measure across the thread where the red arrows are on the image below‭. ‬Refer to the‭ ‬“Actual Major Diameter”‭ ‬column on the chart and then look left to see the Metric and Imperial name of that size‭. ‬

Sizes for BSPT fittings.

To accurately identify any BSPP or BSPT fitting‭, ‬measure from one side of the thread to the other and refer to the chart above‭. ‬For example‭, ‬if the fitting measured 33.249mm across‭, ‬it is a 1”‭ ‬BSP fitting and you would refer to it as a 1”‭ ‬BSP fitting‭. ‬A 2”‭ ‬fitting would measure 59.614mm across the threads and therefore it is called a 2”‭ ‬BSP fitting‭.‬

For British tapered threads‭ (‬BSPT‭), ‬the process is almost the same‭. ‬To determine the threads per inch count‭, ‬you would undergo the same steps as for a parallel thread‭. ‬Using the O.D to identify a tapered fitting is not recommended for the reason that the outer diameter changes throughout the length of the taper‭. ‬However‭, ‬the I.D‭ (‬inner diameter‭) ‬is a good reference point to start from‭. ‬To determine the I.D of a BSP tapered fitting‭, ‬simply obtain a caliper reading from inside the bore of the fitting‭. ‬You can‭ ‬cross-reference the bore size on the following chart with thread pitch and threads per inch to find your desired size and its related specifications‭.‬

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