The Short Answer
BSP (British Standard Pipe) and NPT (National Pipe Thread) are two incompatible threading standards for pneumatic fittings.
– BSP: Parallel threads, used in UK, Europe, Australia
– NPT: Tapered threads, used in USA, Canada, some Asian countries
They look similar but do not fit together. Using the wrong one wastes money and creates leaks.
BSP: British Standard Pipe
BSP threads are parallel—the same diameter from top to bottom. When two BSP fittings connect, the seal comes from a separate O-ring or washer, not from the threads themselves.
Common sizes:
– 1/4 inch BSP (0.571 inch outer diameter)
– 3/8 inch BSP (0.706 inch OD)
– 1/2 inch BSP (0.840 inch OD)
Seal Method: The threads do not seal—a metal washer or O-ring on the fitting does.
Where It is Used:
– UK and Europe (default standard)
– Australia and New Zealand
– Most Parker push-in fittings sold in the UK
– Most Festo regulators and valves in European spec
– PCL quick couplers (UK versions)
Advantages:
– Easy to connect/disconnect (no need to over-tighten for seal)
– Washer is replaceable
– Thread stripping is rare
NPT: National Pipe Thread
NPT threads are tapered—they get narrower toward the end. The seal comes from the thread shape itself.
Seal Method: The cone-shaped thread creates a wedge when tightened. You rely on proper tightness to create the seal.
Where It is Used:
– USA and Canada (default standard)
– Some Asian manufacturers
– Some older UK equipment imported from America
– SIP equipment (some models use NPT)
How to Identify Which You Have
Visually:
BSP:
– Threads look parallel—same width along the entire thread
– Usually has a brass washer or O-ring visible inside the connection
– Fitting does not taper
NPT:
– Threads are visibly tapered—narrower toward the tip
– Threads themselves seal (no washer visible)
– End of fitting is noticeably thinner
Mixing BSP and NPT: What Happens
You can force an NPT fitting into a BSP socket if you are strong enough, but:
– The fit is loose and irregular
– You will get a leak immediately
– The threads may strip on one side
Do not do it. Replace the fitting with the correct standard.
Adapter Fittings
If you need to connect BSP to NPT, adapter fittings exist (usually called BSP to NPT adapters). They are inexpensive (£2-5) and solve the problem cleanly.
However, adapters add connection points, which means more places for leaks. It is better to standardise on one system for your entire workshop.
What Should You Choose?
If You are in the UK: Use BSP. It is the standard, everything is cheaper, and your suppliers stock it in depth. Parker, Festo, PCL—all available in BSP.
Common Mistakes:
– Assuming 1/4 inch is the same regardless of standard (not true—measure or check manual)
– Overtightening NPT threads to get a seal (strips them)
– Forgetting washers on BSP fittings (leads to leaks)
Quality Fittings Make This Easier:
Cheap fittings have loose tolerances. Premium fittings from Festo, ASCO, and John Guest have tight tolerances, so they seal reliably when used correctly.
Building a new system or upgrading old fittings? We stock a full range of BSP fittings and quick couplers from Parker, PCL, Festo, ASCO, and John Guest. All standard sizes, all premium quality.
Call 01474 334537 or browse our fittings and couplers online if you need to specify or replace. We are happy to help.
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