Existing BSP Product
Help & Advice • BSP Thread Identification
The Ultimate Guide to British Standard Pipe Threads
Identify BSPP and BSPT threads correctly, understand nominal BSP sizes, compare thread pitch and outside diameter, and avoid the common mistake of forcing BSP and NPT components together.
BSPP / G Thread
BSPT / R Thread
55° Whitworth Form
Thread Pitch / TPI
Nominal BSP Size
BSP vs NPT
What Is a British Standard Pipe Thread?
British Standard Pipe — BSP — is a family of pipe-thread forms widely used on compressed-air, pneumatic, hydraulic and fluid components in the UK. BSP threads use a 55° Whitworth thread form, and the nominal BSP size does not directly equal the measured outside diameter of the male thread.
The two forms most often encountered are BSPP, which is parallel, and BSPT, which is tapered.
Important: identifying a fitting as “BSP” is only the first step. You still need the nominal size, whether the thread is parallel or tapered, the male/female arrangement and the correct sealing method.
BSPP vs BSPT
BSPP — British Standard Pipe Parallel
The thread diameter remains substantially constant along the threaded length. The pressure seal is normally made by a separate sealing feature such as an O-ring, bonded seal, washer, gasket or sealing face rather than by thread interference alone.
BSPT — British Standard Pipe Taper
The thread tapers and tightens as male and female threaded components engage. A suitable thread sealant is commonly used where specified by the component or system manufacturer.
Original BSPP identification graphic retained from the live article.
Original BSPT identification graphic retained from the live article.
How to Identify a BSP Thread
Do not identify a BSP fitting from visual appearance alone. A reliable identification uses several checks together.
1. Measure DiameterUse callipers across the outside of a male thread or the appropriate reference on a female thread.
2. Check Thread PitchCount threads per inch (TPI) or use a thread-pitch gauge.
3. Check for TaperMeasure at more than one point to determine whether the diameter changes along the thread.
Original BSP thread-identification graphic retained.
BSP Thread Identification Chart
The table below preserves the identification values from the live guide for common workshop sizes. Treat the outside diameter as an identification reference — it is deliberately larger than the nominal BSP size.
| BSP Size | TPI | Major Diameter | Minor Diameter | Tap Drill | Typical Use |
|---|---|---|---|---|---|
| 1/8" | 28 | 9.728 mm | 8.566 mm | 8.70 mm | Small fittings, sensors |
| 1/4" | 19 | 13.157 mm | 11.445 mm | 11.60 mm | Hoses, valves, tool connections |
| 3/8" | 19 | 16.662 mm | 14.950 mm | 15.00 mm | Air tools, manifolds |
| 1/2" | 14 | 20.955 mm | 18.631 mm | 19.00 mm | Compressors, distribution pipework |
| 3/4" | 14 | 26.441 mm | 24.117 mm | 24.50 mm | Larger lines and equipment |
| 1" | 11 | 33.249 mm | 30.291 mm | 30.75 mm | Higher-flow distribution systems |
Tap-drill values need context. Drilling and tapping dimensions should be confirmed against the exact thread standard, tooling and material before machining. Do not use a website table as a substitute for the relevant engineering specification.
BSP Is a Nominal Size — Why 1/2" BSP Measures About 21 mm
This catches out many people. BSP sizes originated from nominal pipe-bore conventions, so the outside diameter of a male thread is not the same as the size printed in its description.
For example, a male 1/2" BSP thread is around 20.955 mm major diameter. Measuring approximately 21 mm therefore points towards 1/2" BSP rather than a “21 mm thread”.
BSP vs NPT
BSP and NPT can look similar but they are different thread systems. BSP uses a 55° Whitworth thread form; NPT commonly uses a 60° thread form. Pitch and profile can also differ depending on size.
- Do not force an NPT component into a BSP port
- Confirm thread type before ordering replacement valves or fittings
- Use an appropriate BSP-to-NPT adaptor when a mixed system genuinely requires one
- Check pressure and medium compatibility of the adaptor as well as its threads
How BSPP Connections Seal
A parallel thread generally needs a dedicated sealing feature rather than relying on tightening the thread until it jams.
Bonded Seal / WasherCommon where the fitting seals against a flat face surrounding the port.
O-RingUsed on components designed with a specific O-ring sealing land or face.
Gasket / Face SealUsed where the component design provides a defined sealing surface.
How BSPT Connections Seal
BSPT threads tighten progressively because of their taper. Depending on the component design and manufacturer instructions, a compatible thread sealant may be required to provide reliable leakage control.
Do not keep tightening a leaking taper thread indefinitely. Over-tightening can crack ports, distort fittings or damage threads. If a joint will not seal, isolate the system and investigate the thread condition, compatibility and sealing method.
All 8 Existing BSP Products — Preserved
The live article contains eight embedded BSP fittings and valves. All eight original product recommendations, routes and product images are retained here.
Existing BSP Product
Tanair 3-8MBVFXF | 3/8" BSP Mini Female x Female Lever Ball Valve - Black
View Product
Choosing Material & Fitting Type
Galvanised Malleable IronCommon for robust fixed compressed-air pipework and threaded distribution components.
BrassWidely used for valves, adaptors and pneumatic components where corrosion resistance and machinability are useful.
Stainless SteelUseful where corrosion resistance, cleanliness or the process environment requires it — subject to the exact system specification.
Pressure rating comes from the actual product. Do not assume a fitting is suitable for a particular pressure merely because it has a BSP thread.
BSP Threads in UK Compressed Air Systems
BSP connections appear throughout compressor installations, including receivers, filters, regulators, valves, hoses, drains, pressure switches and pipework. Correct thread identification reduces the risk of damaged ports, repeated leakage and unnecessary adaptor stacks.
Galvanised Air Line FittingsBrowse sockets, nipples, elbows, tees, bushes, valves and brackets.Browse Range →
Air Hose, Reels & FittingsBuild the flexible side of the compressed-air system around compatible hose and fittings.Browse Hose & Fittings →
Stainless Steel PressfitExplore an alternative fixed-pipework connection system where suitable for the installation.View Pressfit →
Common BSP Thread Problems
Leaks
- Wrong sealing method
- Damaged thread or sealing face
- Incorrectly matched BSPP/BSPT component
- Loose fitting or failed washer/O-ring
Cross-Threading & Damage
- Starting the fitting at an angle
- Forcing unlike thread standards together
- Excessive tightening
- Reusing damaged fittings
Repair damaged pressure connections correctly. Re-cutting or oversizing threads is not a universal repair. If a pressure-containing component has damaged threads, assess whether the approved repair is thread restoration, replacement of the fitting, or replacement of the component.
Maintenance & Leak Prevention
Threaded joints do not need arbitrary monthly dismantling. Instead, include visible connections in the site's normal compressed-air inspection and leak-detection programme.
- Investigate any audible or detected air leak
- Check exposed fittings for corrosion, impact damage or movement
- Replace damaged seals and fittings rather than repeatedly tightening them
- Keep pipework correctly supported so threaded joints are not carrying avoidable mechanical load
- Isolate and depressurise the system before dismantling a joint
British Standard Pipe Thread FAQs
What is the difference between BSPP and BSPT?
BSPP has a parallel thread and normally relies on a separate sealing feature. BSPT has a taper and progressively tightens as the threads engage, with an appropriate sealant used where specified.
How do I know what BSP size I have?
Measure the thread diameter, determine the thread pitch/TPI and check whether the thread is parallel or tapered. Then compare those measurements with a BSP identification chart.
Why does 1/2" BSP measure around 21 mm?
BSP sizes are nominal pipe sizes rather than literal outside-thread measurements. A 1/2" BSP male thread has a major diameter of approximately 20.955 mm.
Can BSP and NPT be screwed together?
They should not be treated as interchangeable. Their thread forms differ, and an appropriate adaptor should be used when converting between standards.
Does BSPT always need PTFE tape?
No single sealing method suits every fitting. Follow the fitting or equipment manufacturer's sealing instructions and use a compatible sealant where required.
Does a BSP thread determine the fitting pressure rating?
No. Thread form alone does not establish allowable working pressure. Use the pressure rating stated for the actual fitting, material and service conditions.
Need Help Identifying a BSP Fitting?
Send a clear photograph plus the outside thread diameter, thread pitch if known, whether the thread appears tapered, and what the fitting connects to. That gives us a much better chance of identifying the correct replacement without guesswork.
Call 0121 753 3330 or use the Contact Us page.
Identify the Thread Before Ordering the Fitting
Diameter + pitch + taper + sealing method is much more reliable than choosing a fitting from appearance alone.
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