Pneumatic port threads: G (BSPP) vs NPT
What to check before you switch a cylinder or valve to a Chinese-manufactured equivalent.
G (BSPP) and NPT port threads are not interchangeable. They use a different thread form and a different way of sealing, so one will not seal reliably in the other. G is the default port thread across Europe, Asia, the Middle East, and most of the world; NPT is the North American default. The practical part: when you move to a Chinese-manufactured equivalent, you usually get to choose the port thread. AirTAC, for example, offers G (BSPP), and on some series a tapered inch thread (PT) or NPT — so you can order the equivalent with a port that matches the fittings your plant already runs, instead of re-plumbing them. Confirm the exact thread callout (the marking that names the thread) for your series and size when you order.
The two standards, side by side
| G / BSPP | NPT | |
|---|---|---|
| Governing standard | ISO 228-1 | ASME/ANSI B1.20.1 |
| Thread form | 55°, parallel (straight) | 60°, tapered |
| Where it seals | off the thread — on a face or bonded washer | on the thread flanks — usually with sealant/PTFE |
| Default region | Europe, Asia, Middle East, most of the world | North America |
Why they do not mix
The two threads are cut to different shapes. A G thread is parallel — the same diameter along its length — with a 55° profile, and the ISO standard is explicit that such threads “are not suitable as jointing threads where a pressure-tight joint is made on the thread.” The seal is made somewhere else: by squeezing a face or a bonded washer outside the thread. NPT is tapered — it gets wider along its length, at about three-quarters of an inch per foot — with a 60° profile, and it seals by the flanks wedging together as you tighten, normally helped by thread sealant or PTFE tape.
Because one seals off the thread and the other seals on it, and because the angles and taper differ, a G fitting in an NPT port (or the reverse) either bottoms out early or never seals — you get a slow leak that is hard to trace. This is not a tolerance you can force. If you have to bridge the two, use a purpose-made G-to-NPT adapter, not more tape.
Other tapered port threads exist — Rc (the ISO 7 internal taper) and PT (its legacy JIS name) are common on cylinders too. The same rule holds: tapered and parallel threads do not seal into each other. Check the exact callout, not just “BSP.”
What this means when you substitute
Port thread is one of the three differences that actually matter when you swap a Western pneumatic part for a Chinese-manufactured one (the other two are the seal compound for your medium and the pressure and duty rating). It is also the easiest to get right, because the Chinese makers publish more than one thread option:
- If your plant is outside North America, your fittings are most likely G (BSPP). Order the equivalent in G and the port matches your existing air lines.
- If your plant standardized on NPT (common in North America), specify it and confirm availability for your series — and never assume a parallel G part will seal into an NPT port.
- Never assume the thread from the model number alone. Confirm the exact port callout on both the part you run today and the equivalent you are ordering.
Frequently asked
Can I screw a G (BSPP) fitting into an NPT port?
No — not reliably. G is a 55° parallel thread that seals on a face or bonded washer outside the thread; NPT is a 60° tapered thread that seals on the thread itself. Forcing one into the other bottoms out or leaks. Match the port thread to the fitting, or use a proper adapter.
Which port thread does AirTAC use?
AirTAC's catalog default is a tapered inch pipe thread, and it lists G (BSPP) as an available option — so you can specify a G port when you order and match the fittings your plant already runs. Confirm the exact callout for your series and size with the supplier before you order.
Is G the same as BSPP?
Yes. 'G' is the ISO 228-1 designation for a parallel (straight) British Standard Pipe thread — what most catalogs call BSPP. It is not the same as BSPT, which is the tapered version.
Which port thread is standard in my market?
G (BSPP) is the default across Europe, Asia, the Middle East, and most of the world. NPT is the North American default. If your plant is outside North America, your existing fittings are most likely G.
Sources
- ISO 228-1 — pipe threads where pressure-tight joints are not made on the threads (thread form, parallel, off-thread sealing).
- ASME/ANSI B1.20.1 — Pipe Threads, General Purpose (Inch): NPT and related inch pipe threads.
- AirTAC SAI (ISO 15552) cylinder catalog — “G thread is available” as a port-thread option.
Need the equivalent in your thread standard?
Send us the part you run today. We confirm the port thread and the rest of the spec match before you commit.