Pneumatic Fitting Thread Standards Explained: BSPP, BSPT, NPT, UNF and Metric
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Choosing the wrong thread is the most common reason a pneumatic quick-connect fitting leaks or fails to assemble. The same nominal size can exist in several standards with different pitches, flank angles and sealing principles, so it pays to identify the thread before ordering. Below is a summary of the standards used on AFD metal quick-connect fittings.
Parallel (straight) threads
Parallel threads hold pressure through a face seal or an O-ring, not through the thread flanks. The common families are:
- BSPP (G, British Standard Pipe Parallel) — 55° flank angle, ISO 228. Sealed by a bonded washer or O-ring.
- UNF / UNS / UN / UNEF (Unified) — 60° flank angle, inch sizes with fine, special, constant and extra-fine pitches.
- NPSM — American National straight mechanical pipe thread, 60°.
- NH — American National hose-coupling thread for fire equipment.
- Metric parallel (M) — designated by outer diameter and pitch, e.g. M5×0.8, M10×1.0.
Taper threads
Taper threads seal on the flanks as they are tightened, usually with a thread sealant; the taper drives the flanks together.
- NPT / NPTF (American National Taper Pipe) — 60° flank angle, NPTF (dryseal) deforms the crests to fill clearances.
- BSPT (R, PT, ZG) — 55° flank angle, ISO 7, taper equivalent of BSPP.
Reference dimensions — parallel threads
| Designation | Flank angle | Pitch (mm) | Major dia. (mm) |
|---|---|---|---|
| 1/8-28 BSPP (PF) | 55° | 0.907 | 9.728 |
| 1/4-19 BSPP (PF) | 55° | 1.337 | 13.157 |
| 3/8-19 BSPP (PF) | 55° | 1.337 | 16.662 |
| 1/2-14 BSPP (PF) | 55° | 1.814 | 20.955 |
| 5/8-14 BSPP (PF) | 55° | 1.814 | 22.911 |
| 3/4-14 BSPP (PF) | 55° | 1.814 | 26.411 |
| 7/16-20 UNF | 60° | 1.270 | 11.112 |
| 7/16-24 UNS | 60° | 1.058 | 11.113 |
| 1/2-16 UN | 60° | 1.588 | 12.700 |
| 9/16-24 UNEF | 60° | 1.058 | 14.288 |
| 1/2-14 NPSM | 60° | 1.814 | 20.904 |
| 3/4-11.5 NH | 60° | 2.209 | 27.242 |
Reference dimensions — taper threads
| Designation | Flank angle | Pitch (mm) | Pitch dia. (mm) |
|---|---|---|---|
| 1/8-27 NPTF | 60° | 0.941 | 9.489 |
| 1/4-18 NPTF | 60° | 1.411 | 12.487 |
| 3/8-18 NPTF | 60° | 1.411 | 15.926 |
| 1/2-14 NPTF | 60° | 1.814 | 19.772 |
| 3/4-14 NPTF | 60° | 1.814 | 25.117 |
| 1/8-28 BSPT (PT) | 55° | 0.907 | 9.147 |
| 1/4-19 BSPT (PT) | 55° | 1.337 | 12.301 |
| 3/8-19 BSPT (PT) | 55° | 1.337 | 15.806 |
| 1/2-14 BSPT (PT) | 55° | 1.814 | 19.793 |
| 3/4-14 BSPT (PT) | 55° | 1.814 | 25.279 |
Practical selection notes
A 1/2-14 NPT male will thread into a 1/2-14 NPSM female for a low-pressure mechanical joint, but never confuse a 1/2-14 BSPP with a 1/2-14 NPT: the pitches differ (1.814 mm on both, but flank angles are 55° vs 60° and diameters differ) and forcing them together ruins both threads. On AFD fittings, R-designated studs are BSPT, G studs are BSPP, and NPT is called out explicitly. Metric studs are marked M-OD, e.g. PC6-M10 accepts 6 mm tube onto an M10×1.0 stud.
If you are unsure which thread a mating port has, measure the outside diameter and count turns per inch, or send a sample to us — AFD Pneumatic supplies fittings in R, G, NPT and metric studs from M5 to 1/2 in, and we will match the thread from a photo or sample.
