British Standard Pipe Thread Sizes, BSP Swimming Pool Pipe

When ordering pipe for UK pools, a new construction or even the refurbishment of a pool constructed 30/40 years ago, unless you have an accurate measurement that states otherwise, swimming pool pipe should always be ordered in Imperial measurements. Standard UK swimming pool pipe for domestic installations is 1.5″ and 2″ where a larger flow rate is required. The specification for Swimming pool pipe used in the vast majority of UK pools (BS/EN1452) can be found on one of our previous articles BSP Swimming Pool Pipe. 

BSP stands for British Standard Pipe, developed from when Britannia Ruled the Waves in manufacturing terms. BSP pipe and thread sizes bear no resemblance to the imperial measurement they are described in. BSP (British Standard Pipe) size refers to the nominal bore diameter of steel pipe coming out of British manufacturing from all them years ago. In swimming pool terms a standard pipe diameter is described at 1.5″ BSP, the outside diameter (OD) of the pipe is actually 48.3mm nowhere near its description size of 1.5 inch. The other main pipe size for domestic swimming pool pipe is 2″ represented by an actual OD size of 60.3mm.

From our own help desk conversations many clients make the mistake of stating they have 2″ pipe on their installation when they placed a ruler and with Rack of Eye (Rock of Eye) guessed the measurement as 2″/50mm. When 1.5″ BSP swimming pool pipe is 48.3mm a close call for the inexperienced eye.

Confusing to say the least but there are many conversion fittings available when the pool equipment being fitted comes with metric pipe or a metric thread set. These days this is particularly apparent for the fittings supplied on a good number of heat pumps. With a short length of 50mm pool pipe, you can normally fit a simple 1.5″BSP/50mm socket to socket on a solvent weld to convert onto what is used in most UK swimming pools.

Understanding BSP Pipe Threads

In swimming pool installations, the most common type is the parallel BSP thread, sometimes referred to as BSPm that is the male type with threads on the outside of the pipe and BSPf the female versions with threads on the inside that the male fits into. With this design, the thread maintains a consistent diameter along its entire length. Very occasionally tapered BSP threads, known as BSPT, are also used, particularly in metal pipework. Unlike parallel threads, tapered threads gradually reduce in diameter towards the end. There is a misconception that PTFE tape should be used on every type of this thread. NO, only the tapered versions, more on this later **

BSP thread sizes are specified in imperial measurements, such as ½”, ¾”, 1”, and upwards. This can be confusing in countries where pipework is measured in metric units. However, many metric swimming pool pipe installation still use imperial thread sizes for their connections. For example, a fitting designed for 50mm pipe may correctly have a 1½” BSP thread. Although this may appear unusual, it is standard practice.

BSP threads have a flank angle of 55 degrees. This is the angle formed between the two sides of the V-shaped thread profile. To establish a seal on a male to female fitting because the threads do not perform a seal in their own right, an appropriate sealing component, such as an O-ring, a flat washer or a bonded sealing washer, is usually necessary. Without a suitable sealing arrangement, BSP threads alone will not normally provide a reliable watertight joint.

**Tapered BSPT threads work differently because the taper allows the mating threads to tighten against one another and form a seal. A small amount of PTFE tape or another suitable thread-sealing compound is generally applied to help create a secure, leak-resistant connection. Top tip, do not overtighten BSPT or apply large multiple layers of PTFE tape, it is so easily done cracking the outer plastic of the thread housing. For some really technical stuff you can read the wikipedia page on this subject.

BSP and NPT Threads: Why They Are Not Interchangeable

NPT is a tapered thread system, whereas BSPm and BSPf are parallel. Although both standards use imperial size designations, their thread profiles and specifications differ. Consequently, they should not be treated as interchangeable. The difference in thread design makes it difficult to achieve a dependable seal when the two are connected. Attempting to compensate by tightening the fittings excessively can damage the threads or crack the plastic components.

For this reason, BSP and NPT fittings should not be mixed. Always identify the thread standard correctly and use compatible fittings and sealing components to ensure a safe, reliable connection.