British Standard Pipe (BSP) thread is the most common pipe thread standard used in Ireland, the UK, and most of Europe. There are two types: BSPP (parallel, also known as G thread) and BSPT (tapered, also known as R thread). Both use a 55° flank angle, distinguishing them from the US NPT standard (60°). BSPP threads seal with a washer or O-ring at the face; BSPT threads seal on the thread taper with PTFE tape or thread sealant. Pneumatics Group provides a free BSP thread identification reference and supplies BSP fittings for pneumatic and engineering applications across Ireland.
British standard pipe Thread fittings are amongst the most popular threads in the world today.
Here is how to identify a BSP thread.
– Parallel (BSPP)
– Tapered (BSPT)
The thread flank angle for both tapered and parallel British threads is 55°.
It is fairly common for a tapered BSP thread to be mistaken for an NPT thread. The way to tell the difference between the two is the flank angle.
The flank angle for NPT is 60° compared to the BSP tapered which is 55° (this can be verified with a thread gauge).
BSP threads are measured in imperial measurements such as 1/8, 1/4, 1/2, 3/4 etc.
BSP parallel threads commonly seal via a 30° chamfer on the male thread to a 30° recessed cone inside the female thread (swivel only).
If it is a port application an o-ring and a washer or bonded washer are needed to achieve a proper seal for parallel threads.
For tapered BSP threads, a seal is acquired via thread wedging with additional support from thread dope or Teflon tape.
As previously mentioned, it is possible for a male BSPT (tapered) to fit into a female BSPP (parallel), so long as the female thread is fixed and not swivel (this is due to the recessed cone seat).
1) Measure the O.D (Outer diameter) of the BSP thread.
2) Take the O.D measurement in inches and subtract 1/4 (.25″).
For example, a BSP parallel male thread measures out to O.D 1″.
Subtracting 1/4 gives a thread size of 3/4 also known as “dash” 12.
Once you have the thread size, you then need to determine the number of threads per inch to verify that it is a BSP fitting.
You can do this by counting the number of thread crests over a 1/4″ length and then multiply this by 4 to get the number of threads per inch.
1) Multiply 3.5 by 4 to get 14. This is the number of threads per inch
2) Combine the thread size with the number of threads per inch and refer them to the following chart to find your desired size and its relative specifications.
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Using the wrong thread standard is one of the most common and expensive mistakes in pneumatic and hydraulic system installation. A BSP fitting forced into an NPT port will cross-thread. A BSPT male sealed with only a face washer will leak. Getting thread identification right before ordering saves time, prevents leaks, and avoids system damage.
This guide explains the difference between BSPP and BSPT, how to identify and measure BSP threads, and how to distinguish BSP from NPT. Use the reference tables below, or contact our team for identification help.
Feature | BSPP (Parallel) | BSPT (Tapered) |
Also known as | G thread (ISO 228) | R thread (ISO 7) |
Thread diameter | Constant along length | Increases/decreases along length |
Sealing method | Face seal O-ring, bonded washer, or soft washer | Thread engagement PTFE tape or thread sealant |
Male to female match | BSPP male into BSPP female (parallel to parallel) | BSPT male into BSPP or BSPT female (tapered-to-either) |
Common use in Ireland | Pneumatic fittings, instrument ports, hydraulic connections | Pipe installations, compressed air mains, water systems |
Flank angle | 55° | 55° |
BSP and NPT (National Pipe Thread, used in North America) threads look similar and are sometimes confused especially in imported machinery.
Feature | BSP (BSPP/BSPT) | NPT (US Standard) |
Flank angle | 55° | 60° |
Thread form | Rounded peaks and valleys (Whitworth) | Flat peaks and valleys |
Taper rate (tapered) | 1 in 16 (BSPT) | 1 in 16 (NPT) similar but not compatible |
Compatibility | BSP and NPT are NOT interchangeable | Do not mix in high-pressure systems |
How to confirm | Use a thread gauge or measure flank angle with a gauge | Count TPI and measure OD against chart |
Important: BSP and NPT threads appear almost identical at low magnification and have the same nominal sizes in many dimensions but they will not seal reliably in high-pressure systems. Always confirm thread type before connecting.
Use a vernier calliper to measure the thread OD in millimetres or inches.
BSP nominal size = OD (in inches) minus ¼. For example, if OD = 1 inch, the BSP size is ¾ (also written as G3/4 or R3/4).
Count the thread crests over ¼ inch and multiply by 4 to get TPI. Match to the BSP thread chart below.
Parallel (BSPP) threads have a constant diameter. Tapered (BSPT) threads grow or shrink. Run a ruler along the thread if diameter changes, it’s tapered.
For critical applications, use a 55° thread gauge to confirm BSP flank angle. This rules out NPT (60°).
BSP Size | OD (mm) | TPI | Common Application |
1/8″ | 9.73 | 28 | Instrument ports, small fittings |
1/4″ | 13.16 | 19 | Pneumatic fittings, solenoid valve ports |
3/8″ | 16.66 | 19 | Pneumatic cylinders, FRL units |
1/2″ | 20.96 | 14 | Air lines, compressed air mains |
3/4″ | 26.44 | 14 | Air receivers, larger pipe systems |
1″ | 33.25 | 11 | Industrial compressed air mains |
1¼” | 41.91 | 11 | Large-bore pipework |
1½” | 47.80 | 11 | Plant air mains, compressor outlets |
BSPP stands for British Standard Pipe Parallel. It is a parallel-thread pipe standard defined by ISO 228, also commonly referred to as G thread. Sealing is achieved at the face using a washer, O-ring, or bonded seal not on the threads themselves.
A tapered (BSPT) male thread can be fitted into a parallel (BSPP) female port if the female is fixed (not swivel). It will not seal correctly in a swivel female fitting. In any case, thread sealant or PTFE tape is required. For reliable sealing, match BSPT to BSPT or use appropriate adapters.
No. While they share similar nominal sizes, BSP (55°) and NPT (60°) have different thread angles and will not seal reliably. Using NPT fittings in BSP ports (or vice versa) risks cross-threading, leakage, and system failure particularly in compressed air and hydraulic systems.
“G thread” is the ISO 228 designation for BSPP (British Standard Pipe Parallel). G¼, G½, and G¾ are commonly used on pneumatic component port markings in Europe and Ireland.
Pneumatics Group stocks a comprehensive range of BSP fittings, adapters, reducers, and union connectors in both BSPP and BSPT for pneumatic and engineering applications available from our Dublin warehouse with same-day dispatch.