Technical Bulletin: Plastic pipe threaded fittings

1. Thread types

Plastic pipe systems such as PVC and ABS have the advantage of being able to be quickly and easily assembled, using solvent cement that is painted on the pipe and socket. There may however be occasions when a threaded fitting is required. Such a situation often occurs when connecting to pumps or other equipment or when transitioning from one material to another (such as plastic to metal).

BSP Threads

Nearly all metric and most European made imperial threaded pvc pipe fittings, have a BSP Parallel thread on them. The “BSP” denotes that the set and depth of the thread conform to “British Standard Pipe” thread. An additional “P” is often used as a suffix (BSPP), to denote the thread is a parallel one rather than a tapered one. Additional suffixes of either “M” or “F” are also sometimes used. These denote whether the thread is a male (i.e. thread on the outside) or female (i.e. thread on the inside) fittings.

NPT Threads

Apart from BSP, the other commonly found thread in plastic pipe systems is NPT, an American standard (National Pipe Taper), which is only usually found on plastic pipe parts that originate in the USA.

Comparison of BSPP and NPT threads

BSPP and NPT threads are generally not compatible. The first hurdle we come to is that one is parallel and the other tapered (a tapered thread is a conical shape, whereas a parallel is the same diameter all the way along). As such, even if the pitch and thread depth were the same, a male parallel will only go into a female tapered by a single thread and will not be secure. A male taper will usually go into a female parallel about 40-50% of the way and can, as long as a thread sealant is correctly used, be regarded as being generally secure. 

Pipe Size	Pitch  (Threads/Inch)
	NPT/NPS	BSP
1/16"	27	---
1/8"	27	28
1/4"	18	19
3/8"	18	19
1/2"	14	14
5/8"	---	14
3/4"	14	14
1"	11 1/2	11
1 1/4"	11 1/2	11
1 1/2"	11 1/2	11
2"	11 1/2	11
2 1/2"	8	11
3"	8	11
3 1/2"	8	11
4"	8	11
5"	8	11
6"	8	11
8"	8	---

The above table gives the pitch (the number of threads per inch) of NPT and BSPP threads. As can be seen, the only two sizes that are compatible are ½” and ¾”. All other sizes have different pitches and so will thread together. The ½” and ¾” sizes will connect, but only if the male part is NPT and the female is BSPP, and then only expect the thread to insert to about 40-50% of the whole thread.

Connecting different thread types

The preferred option to connect different thread types is to use one or more adaptor fittings. For example if it is required to connect a 1” BSPPM (male) to a 1” NPTF (female), the correct method would be to us a 1” BSPPF : Plain socket, and a 1” NPTM : Plain nipple. This would extend the length of the fitting, which in some cases may cause issues, however it ensures that a system is correctly fitted, and suitable for its design pressure specification.