Pipe Threads | NPT vs BSP

Threaded connections are one of the common ways of connecting pipes.  Threads are either “male” or “female” and most people (who let’s all hope have enough carnal knowledge) can readily guess which is which.  For the benefit of the rest of the population, it’s like this:  Threads on the outside is male, threads on the inside is female.

Male Threaded Pipe Ends

Female Threaded Pipe Ends

Most of the world will use either NPT (American Standard Pipe Taper Thread, to ASME B1.20.1) or BSP (British Standard Pipe Thread to BS21).   By the way, BSP threads are also interchangeable with ISO, DIN and JIS.  NPT and BSP are not compatible and should not be coupled together (ever).  This is especially true for the smaller threads that are compatible to each other– you will notice a slight resistance when turning them.  Compatible threads should feel easy when you’re turning them during connection.  Having to put a little force to screw them together is a sign that you have incompatible thread standards, and doing so with just damage the threads permanently.

So, NPT or BSP?  It’s really up to you.  Here’s a suggestion:  if you tend to use American standards (for your designs and in your plant) or if your client’s plant is mainly using American standards, then perhaps it’s wise to go for NPT threads.  Likewise, it’s BSP if for example your equipment is already built to BS 2790 (British Standards for welded shell boilers).