Thread mismatch is a common cause of delayed orders — and of leaks that surface months after a panel ships. Our factory can machine NPT, G (BSPP), ZG/R (BSPT) and metric ends, but the RFQ must state which one you need: the two big families seal in completely different ways and are not interchangeable.
Key takeaways
- NPT (ASME B1.20.1) is a 60° thread tapered 1:16 — ¾ inch per foot — that seals on its own flanks using PTFE tape or liquid sealant; G/BSPP (ISO 228-1) is a 55° parallel thread that never seals on the thread and instead seals on a bonded washer or O-ring at the face.
- NPT and G are not interchangeable: some sizes share the same threads-per-inch, but the 60°-versus-55° flank mismatch galls the threads and the joint weeps at line pressure.
- Sealant belongs on tapered threads only — dope on a G thread can keep the fitting from reaching proper torque and leave the face seal under-compressed.
Two sealing philosophies
A tapered thread is its own seal. As an NPT male screws into an NPT female, the 1:16 taper wedges the flanks together, and the remaining spiral leak path is filled with sealant — PTFE tape or pipe compound. A parallel thread is only a fastener: G threads run in with clearance and spin home by hand, and the pressure-tight seal is made separately by crushing a bonded seal (a metal washer with a vulcanized elastomer ring), an O-ring in a recess, or a flat gasket against a machined face. Printable versions of both thread families live on our NPT thread chart and BSPP thread chart sheets.
| NPT | G (BSPP) | |
|---|---|---|
| Governing standard | ASME B1.20.1 | ISO 228-1 (ex-BS 2779) |
| Thread form | 60° | 55° Whitworth |
| Taper | 1:16 (¾ in per ft) | none — parallel |
| Seals on | thread flanks + sealant | face: bonded washer / O-ring |
| Sealant needed | yes | no — it can spoil the face seal |
| Typical region | North America | Europe, Asia, export skids |
How to tell what you have
Read the marking first: “NPT”, “G1/4”, “Rp”, “Rc” or “R” on the part or drawing settles it. With no marking, run calipers along the thread length — a male NPT’s diameter visibly shrinks toward the end; a G male’s does not. A pitch gauge helps, but similar TPI does not prove compatibility. A useful feel test: a G male spun into a matching G female turns home with clearance, while NPT starts easy and wedges within a few turns. For the full bench routine — pitch gauge, calipers, and an OD + TPI lookup table for every common size — see how to identify NPT vs BSP threads.
Why mixing leaks
The traps look compatible and are not. NPT into G: the taper wedges against parallel threads until something yields, but there is no designed seal path — the joint feels tight and weeps under pressure. G into NPT: the parallel thread never wedges, and the face has nowhere to land. Even NPT into BSPT — both tapered at the same 1:16 — fails, because the 60° American flank and the 55° Whitworth flank ride on crest contact instead of the faces, galling on assembly and leaking under temperature or vibration. On stainless fittings the risk compounds: crest contact under assembly torque is exactly the condition that galls stainless threads — see why stainless steel threads gall and how to prevent it for the mechanism and the assembly-lubricant countermeasure. Cross-threaded “but it went in” joints are the most common thread callback we see on imported panels.
The taper cousin: R / ZG (BSPT)
British taper pipe thread — R for the male, Rc for the female, ZG on many Chinese drawings — follows ISO 7-1 (older references cite BS 21 or EN 10226-1). It seals on the thread with sealant like NPT, and a G male can legitimately assemble into an Rc female where the design intends a thread-and-face combination — but R and NPT still do not mix. If a drawing says only “BSP,” ask which variant before it becomes a purchase order.
Side by side, the three threads you are most likely to meet on a stainless fitting:
| NPT | G (BSPP) | R / ZG (BSPT) | |
|---|---|---|---|
| Thread form | 60° Unified, tapered | 55° Whitworth, parallel | 55° Whitworth, tapered |
| Standard | ASME B1.20.1 | ISO 228-1 | ISO 7-1 (older: BS 21, EN 10226-1) |
| Taper on diameter | 1:16 | none (parallel) | 1:16 |
| Seals on | Thread flanks (interference) plus sealant tape or paste; NPTF is a separate dryseal spec, not a no-sealant guarantee | End face — bonded sealing washer or O-ring; the threads themselves carry no seal | Thread flanks (interference) plus sealant |
| Typical pairing | NPT male into NPT female | G male into G female, washer under the face | R male into Rc female (R into Rp parallel female is also used) |
| Where you meet it | North American panels, compressors, utility skids | Europe, UK, Asia-Pacific, Middle East OEM equipment | Same G markets, wherever a drawing calls out taper |
| Mixing with either of the others | Engages a few turns at best; cannot be relied on to seal | Feels tight, weeps under pressure — no designed seal path | Engages a few turns, jams on crest contact |
| Spot-check at 1/2 in | 14 TPI, major ≈ 0.840 in (21.34 mm) | 14 TPI, major ≈ 20.955 mm | 14 TPI, major ≈ 20.955 mm |
| Spot-check at 1/4 in | 18 TPI, major ≈ 0.540 in (13.72 mm) | 19 TPI, major ≈ 13.157 mm | 19 TPI, major ≈ 13.157 mm |
The 1/2 in row is the trap that catches buyers: 14 TPI on both sides invites the conclusion that NPT and BSP interchange there. The flank angle and the major diameter still differ — the threads are not compatible at any size.
Regional conventions worth knowing
North American panels, compressors and utility skids run NPT almost exclusively. Europe, the UK, Australia, India, the Middle East and most Asian OEM equipment run G or R, with metric instrumentation ports (M5–M24 style) layered on top. Export skids we build routinely mix all three — a metric compression body on one end and an NPT or G male on the other — which is normal, provided the RFQ names each end explicitly.
What to send for a fast factory quote
- Tube OD or pipe size, and the thread standard on each end — e.g. “6 mm tube × 1/4 NPT male” or “G1/2 male × Rc1/2 female”
- Material grade (SS304 / SS316 / SS316L)
- Whether bonded seals or thread sealant should be included
- Quantity and destination
Mixed threads are normal for export OEM work — list both ends clearly and we confirm stock or machining lead time. Start from the compression-to-male-connector range for tube-to-thread bodies, or the female threaded ball valve and straight female needle valve lines for threaded valves, then request a factory quote with both ends spelled out.
FAQ
Can I seal NPT with a gasket instead of tape? No. NPT has no sealing face — the seal is thread flank interference plus sealant. A gasket on an NPT joint sits unloaded and does nothing; skipping the sealant leaves the spiral leak path open.
Is G thread the same as BSP? Almost. BSPP — the parallel variant — is G, governed by ISO 228-1. BSP also includes BSPT, the tapered variant (R/Rc per ISO 7-1), which seals on the thread like NPT but with a 55° form. “BSP” alone on a drawing deserves a clarifying question.
Why did my G-thread joint leak when I used plenty of PTFE tape? Because the seal is at the face, not the thread. Tape or compound on G threads lubricates the assembly, the fitting reaches a false torque before the bonded washer compresses, and the face seal stays loose. Use a bonded seal or O-ring, clean and dry.
Are NPT and BSPT interchangeable if the TPI matches? No. The 1:16 tapers match numerically, but the 60°-versus-55° flank angle makes the threads ride on their crests, gall on make-up, and leak under load or vibration. Convert with a proper adapter — never force the pair together.
How do I convert between NPT and G on an existing panel? With a male-female adapter machined for exactly the two standards involved, such as our male-female straight adapters made to order. Adapters are cheap; cross-threaded valves and remade panels are not.
