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BSP is the acronym for “British Standard Pipe Thread”. BSP threads are among the most common type in the world as far as pipework systems are concerned. Most plastic pipework systems use a parallel type of BSP thread (often referred to as BSPP), that is to say the diameter of the thread remains the same for the entire length. Tapered BSP threads (BSPT) are sometimes found in metal pipe systems. With tapered threads the diameter of the thread decreases towards the tip. BSP threads are expressed in imperial sizes such as; ½”, ¾”,1”, 1 1/4” and so on. This can often cause confusion in parts of the world where metric pipe systems are used. Most metric plastic pipe systems use imperial threads. It can sometimes seem odd to be specifying for example a 50mm pipe fitting with 1 ½” threads, but this would be correct. There is no such thing in the plastic pipe world as a 50mm thread.

A BSP threads have a 55 degree flank angle, which is to say the angle of the “v” formed by two threads. The end of a BSP thread is designed to sit on a seat (usually 30 degrees) and seal against it. If the application does not use and end seal and is a “port”, then an O-Ring , washer or a bonded washer will be required. BSPP threads will not otherwise seal properly.
Tapered threads are designed to seal by the nature of the threads binding into each other. For these threads a small amount of PTFE tape, or a similar thread sealant product, should be applied.

We will concentrate on BSPP threads in this article, as these are the types most commonly found in plastic pipe systems.
To determine the correct size of a BSP thread you must measure the outside diameter of the thread and subtract ¼”. So a thread that measures ¾” diameter is actually a ½” thread etc. Confusing I know! To then check that your thread is in fact a BSP one, count the number of thread crests over a ¼” (6mm) length of the thread. Multiply this by four to get the threads per inch, and then refer to the chart below. (Tip – for threads larger than 1“ it is easier to measure the number of thread crests over ½” and multiply by two.)

At the plastic pipe shop we are often contacted by people who have other threads in their plastic pipe systems. These are usually NPT (National Pipe Thread), which is a North American standard. This is a tapered thread and, despite what some say, these are NOT compatible with BSP threads. The ½” NPT is often touted as being the same as ½” BSPP but it is not. One is a taper and the other a parallel. Achieving a good seal, without resorting to over-tightening and cracking the fittings, is very difficult and should be avoided.