Sometimes in our industry, we are faced with a choice between two things that do the same job, and when that happens, people always want to know which one is best. Such is the case with NPT vs BSP pipe threads.
So, what is the difference between NPT thread and BSP thread? Let’s look at the names first.
BSP stands for British Standard Pipe, and it is found in many countries around the world due to the reach and influence of the old British Empire, which accounts for such things in Australia as the fact that we drive on the left side of the road and the driver is on the right-hand side of the car. The Brits may not be running things here any more, but the influence remains.
NPT stands for National Pipe Thread, and the nation in question is the USA, which has had global reach for many years now.
There are two key differences in this matter of BSP thread vs NPT thread: the thread angle and the shape of the peaks and troughs. BSP has a 55-degree angle, and the peaks and troughs are rounded. With NPT, it’s 60 degrees, and the peaks and troughs are flattened and more angular.
There is No Right and Wrong
Both NPT thread and BSP thread work perfectly well and have been doing a fine job in their rather specialised field for many years. It’s a simple matter of creating and maintaining a tight connection that will prevent leakage in pipe joints, and the situation is well under control in the USA, just as it is in Australia, New Zealand and the UK.
Both BSP threads and NPT threads do the job they’re supposed to, and generally speaking, if there is any leakage problem in this respect, it will be down to the quality of the parts or the workmanship.
NPT Thread and BSP Thread are Not Interchangeable
While it is entirely down to the individual which type they use in practice, there is no difference between NPT thread and BSP thread as regards efficiency. What you can’t do is use them together. They’re the same kind of thing and, at a glance, they might look identical, but those slight differences make it impossible for them to fit together precisely. You have to use one or the other.
Why is there a Difference Between NPT Thread and BSP Thread?
One day in the distant future, there may be complete standardisation of measurements around the world, but even then, NPT and BSP threads would not be compatible because of the nature of the peaks and troughs.
While Australia adopted the metric standard in the 1970s and it became compulsory in 1988, and the UK did something similar around the same time, although the US nominally accepted the metric idea, it met with more resistance among the general population and has so far not been pushed through as it has in most other countries. This may seem odd when, long before ours, their currency used cents, 100 to the dollar, a metric division, whereas in the UK and Australia we used to labour under the old random units of pounds, shillings and pence, with 12 pence to the shilling and 20 shillings to the pound.
Be that as it may, NPT thread is a hangover from earlier times, and as things stand at the moment, the Australian workforce can choose either NPT or BSP but cannot mix and match.
How To Tell BSP Thread from NPT Thread
If you find yourself working on an existing system and are not sure whether it features NPT or BSP threads, careful testing by hand can help you find out. Get a finger in and feel the threads: sharp indicates NPT and smoother, more rounded suggests BSP. You could also measure the threads per inch and download a table. Alternatively, you can make the decision to replace the threads and standardise them to your liking.