In this article:
We discuss the technical aspects of the BSP Thread standard
We look at: BSPP & BSPT, Thread Counts (TPI) & Pitch
Overview of BSP Thread Standard
British Standard Pipe (BSP) is the most common thread standard for plumbing and pipe fitting in the UK and is an international standard that is also adopted in varying degrees in other countries around the world.
The BSP thread is also known as the Whitworth 55° Thread Form.
There are two types of BSP thread available:
One is
Parallel threads (BSPP) and the other type is
Tapered Threads (BSPT)
BSPT is BRITISH STANDARD PIPE TAPER THREAD – Thread form per AS 1722.1, BS 21, ISO 7.
BSPP is BRITISH STANDARD PIPE PARALLEL THREAD – Thread form per AS 1722.2, ISO 228.
BSPP - British Standard Pipe Parallel Thread
Also known as British Standard Pipe Mechanical Thread (BSPM) or British Standard Pipe Fitting Thread (BSPF)
BSPP Female & Male Threads are both parallel.
i.e, Threads are the same diameter along the entire length of the fastener.
BSPP Thread is used for
long screw threads, where a pressure tight joint is achieved by the compression of a soft material, such as an 'o' ring, seal or gasket, between the end faces of components, such as in tank connectors, where backnuts are threaded on to a threaded body to compress a seal.
BSPT - British Standard Pipe Tapered Thread
The diameter of the thread will increase or decrease along the length of the male thread causing it become increasingly tighter as it is threaded into the female counterpart.
BSPT Thread is used for
jointing threads
where a pressure-tightness is achieved by the mating of the male and female threads together.
International standards require that all female threads be parallel and the male thread be tapered.
The female thread may also be tapered but this is
extremely uncommon.
Thread Count (TPI) & Thread Pitch
Thread Count
Thread Count is the number of threads per inch measured along the length of the fastener, it is expressed as TPI (Threads per inch)
In general smaller fittings have finer threads so the thread count is
higher
Thread Pitch
The thread pitch is the distance between threads expressed in millimetres.
In general smaller fittings have a finer threads so the pitch is lower
Properties of BSP Thread Standard
- Symmetrical V-thread in which the angle between the flanks is 55° (measured in an axial plane)
- One-sixth of this sharp V is truncated at the top and the bottom
- The threads are rounded equally at crests and roots by circular arcs ending tangentially with the flanks
BSP Thread Chart
This chart shows the Thread Pitch, Number of threads per inch and the height of thread for each designation in the BSP thread standard.
The differences between BSPP & BSPT are not in the thread dimension themselves, but rather the tapering diameter along the length of the fixing, so the figures provided here are applicable to both Paralllel and Tapered forms of BSP.
| Designation of Thread | TPI | Pitch in mm (P) | Height of thread in mm (H) |
|---|---|---|---|
| 1/16" BSP | 28 | 0.907 | 0.581 |
| 1/8" BSP | 28 | 0.907 | 0.581 |
| 1/4" BSP | 19 | 1.337 | 0.856 |
| 3/8" BSP | 19 | 1.337 | 0.856 |
| 1/2" BSP | 14 | 1.814 | 1.162 |
| 5/8" BSP | 14 | 1.814 | 1.162 |
| 3/4" BSP | 14 | 1.814 | 1.162 |
| 7/8" BSP | 14 | 1.814 | 1.162 |
| 1" BSP | 11 | 2.309 | 1.479 |
| 1 1/8" BSP | 11 | 2.309 | 1.479 |
| 1 1/4" BSP | 11 | 2.309 | 1.479 |
| 1 1/2" BSP | 11 | 2.309 | 1.479 |
| 1 3/4" BSP | 11 | 2.309 | 1.479 |
| 2" BSP | 11 | 2.309 | 1.479 |
| 2 1/4" BSP | 11 | 2.309 | 1.479 |
| 2 1/2" BSP | 11 | 2.309 | 1.479 |
| 2 3/4" BSP | 11 | 2.309 | 1.479 |
| 3" BSP | 11 | 2.309 | 1.479 |
| 3 1/2" BSP | 11 | 2.309 | 1.479 |
| 4" BSP | 11 | 2.309 | 1.479 |
| 4 1/2" BSP | 11 | 2.309 | 1.479 |
| 5" BSP | 11 | 2.309 | 1.479 |
| 5 1/2" BSP | 11 | 2.309 | 1.479 |
| 6" BSP | 11 | 2.309 | 1.479 |
All the dimensional figures shown in this chart are subject to tolerance ranges.
NOTE ON MATERIAL EXPANSION
All materials including plastic are susceptible to expansion and contraction with varying temperatures causing figures to be subject to variations.
Each material will expand and contract at different rates, plastic is more susceptible than most metals.
Summary:
• BSP is the most common thread standard for plumbing and pipe fitting in the UK.
• BSPT is most common for joints where pressure resistance is achieved by the mating of threads, such as plugs.
• BSPP is used when pressure resistance is achieved by compressing a seal or a gasket.
• If a fitting has BSPT thread it is always the male counterpart that is tapered, making it compatible with all BSP female threads of the same size designation.
All information provided in this article is intended as a preliminary guide only. This chart does not constitute actual advice and
Pipe Warehouse UK is not responsible for any issues arising from the use or reliance on the information contained in this article.