How it connects
Screws onto a male pipe thread; parallel threads give only mechanical retention. The seal is made by a flat washer/gasket (e.g. drain tailpieces, slip nuts) or by a 30° internal cone seat in hydraulic swivel nuts.
Male and female halves
Female (internal) NPSM, typically in a nut or swivel. Mates with male NPSM or male NPT of the same nominal size.
Usually connects to
- Matching male NPSM thread with 30° seat
- Matching male NPT shower arm/fitting where a compatible washer/cone seat makes the seal, rather than the loose-fitting threads
- Matching 30° hydraulic cone-seat adapter
Common sizes
- NPSM nominal ranges are listed from 1/8–6 in, with many common lines ending at 4 in.
- Pitches: 1/8-27; 1/4 and 3/8-18; 1/2 and 3/4-14; 1–2 in at 11.5 TPI, including 1-1/4 and 1-1/2; larger examples use 8 TPI.
- 1/2-14 is common for shower heads.
Where it is used
hydraulic hose swivel fittingsplumbing drain tailpieces and fixture connectionsconduit/locknut hardwaregas and fluid fittings with gasket sealshydraulic swivel adaptersshower heads and armspressure washer and hose fittingsfluid power
Variations
- NPSM swivel nut with 30° cone seat
- washer-sealed NPSM nut
- NPSM female swivel with 30° seat
How to recognise it
Internal parallel thread (constant diameter) with pipe-thread pitch (27/18/14/11.5/8 TPI), usually in a hex swivel nut with a 30° cone seat inside, or a slip/drain nut used with a flat washer.
Often confused with
| Confused with | How to tell them apart |
|---|---|
| NPT pipe thread | NPT is tapered 1:16 and seals on the threads with sealant; NPSM is parallel and needs a washer or cone seat. NPT is tapered and seals on the threads; NPSM is parallel and needs a washer or seat, often on a swivel nut. |
| NPSC / NPSL | Same B1.20.1 family; NPSC is for couplings sealed with sealant, NPSL for locknut fits with tighter/looser classes. NPSM is the free-fitting mechanical class. |
| NPSH (garden hose GHT) | NPSH/GHT uses different pitch (e.g. 3/4-11.5) and larger diameter than 3/4 NPSM. |
| BSPP (G thread) | BSPP has 55° Whitworth form and different TPI at most sizes. BSPP has 55° Whitworth form and different TPI at most sizes (e.g. G1/2 is 14 TPI but ~20.96 mm vs NPSM ~21.0 mm, nearly interchangeable but not equal). |
Also called
straight pipe thread for mechanical jointsNPT female swivel (hydraulics)NPSM female swivelFPS (female pipe swivel)NPS mechanical thread
Good to know
Sample part '1/2 RMC female locknut' is arguably NPSL (locknut thread), not NPSM; check before assigning. Images show male NPT x female NPSM swivel adapters, female end mostly hidden.
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