ISO 7-1 · BS 21
A 55 degree Whitworth tapered thread that seals on the thread flanks, the British counterpart to NPT. Interchangeable in size with BSPP but not in behaviour.
10 sizes55 degree flanksTapered thread, 1 in 16Tapered thread, sealed on the thread
No cone, no groove, no flat sealing face - just a thread that visibly narrows towards the end of the fitting.
Where it seals: The thread flanks wedging together, normally with a thread sealant or PTFE tape. The seal is the thread.
A tapered thread seals by wedging, so it strips the port if it is over-tightened chasing a leak. Use sealant or PTFE tape, and stop at the published turns past finger tight rather than at a feel.
Sized by trade size, same nominal series as BSPP.
Where you will find it: European and Asian plumbing and pneumatics, and the tapered ports of imported hydraulic components.
Also searched as: BSPT, R thread, Rc, BSP taper, BS 21, PT thread.
All sizes
| Size | Thread | TPI / pitch | Basic major Ø | Measured across crests | Tube / trade size | Working pressure |
|---|---|---|---|---|---|---|
| R 1/16 | R 1/16 (28 TPI, 55 degree, taper 1:16) | 28 TPI (0.907 mm pitch) | 7.72 mm / 0.304 in | Not published | 1/16 in trade size | Manufacturer-specific |
| R 1/8 | R 1/8 (28 TPI, 55 degree, taper 1:16) | 28 TPI (0.907 mm pitch) | 9.73 mm / 0.383 in | 9.5 mm to 10.1 mm | 1/8 in trade size | Manufacturer-specific |
| R 1/4 | R 1/4 (19 TPI, 55 degree, taper 1:16) | 19 TPI (1.337 mm pitch) | 13.16 mm / 0.518 in | 12.8 mm to 13.6 mm | 1/4 in trade size | Manufacturer-specific |
| R 3/8 | R 3/8 (19 TPI, 55 degree, taper 1:16) | 19 TPI (1.337 mm pitch) | 16.66 mm / 0.656 in | 16.3 mm to 17.1 mm | 3/8 in trade size | Manufacturer-specific |
| R 1/2 | R 1/2 (14 TPI, 55 degree, taper 1:16) | 14 TPI (1.814 mm pitch) | 20.95 mm / 0.825 in | 20.4 mm to 21.5 mm | 1/2 in trade size | Manufacturer-specific |
| R 3/4 | R 3/4 (14 TPI, 55 degree, taper 1:16) | 14 TPI (1.814 mm pitch) | 26.44 mm / 1.041 in | 25.9 mm to 27 mm | 3/4 in trade size | Manufacturer-specific |
| R 1 | R 1 (11 TPI, 55 degree, taper 1:16) | 11 TPI (2.309 mm pitch) | 33.25 mm / 1.309 in | 32.6 mm to 33.9 mm | 1 in trade size | Manufacturer-specific |
| R 1 1/4 | R 1 1/4 (11 TPI, 55 degree, taper 1:16) | 11 TPI (2.309 mm pitch) | 41.91 mm / 1.65 in | 41.1 mm to 42.6 mm | 1 1/4 in trade size | Manufacturer-specific |
| R 1 1/2 | R 1 1/2 (11 TPI, 55 degree, taper 1:16) | 11 TPI (2.309 mm pitch) | 47.8 mm / 1.882 in | 47 mm to 48.5 mm | 1 1/2 in trade size | Manufacturer-specific |
| R 2 | R 2 (11 TPI, 55 degree, taper 1:16) | 11 TPI (2.309 mm pitch) | 59.61 mm / 2.347 in | 58.6 mm to 60.5 mm | 2 in trade size | Manufacturer-specific |
No working pressure is published for this standard. No two independent sources in the register give the same figure under conditions they both state, and a generic number would be a decoration rather than a rating. Read it from the manufacturer's catalogue.
On paper
Dimensions are as defined by ISO 7-1, BS 21. FittingKey is an independent reference and is not affiliated with, endorsed by or approved by any standards body or fitting manufacturer.
Every value on this page was checked against 5 independent published sources:
| Column | Sources | What they gave |
|---|---|---|
| Basic major diameter | iso-7-1, machiningdoctor-bsp | Read directly from ISO 7-1:1994 Table 1, where the major diameter is defined AT THE GAUGE PLANE, and matched by an independent reproduction. ISO 7-1 lists no 5/8 and no 7/8 size. |
| Measured male thread diameter | gerrard-id, adaptall-british, gates-idchart | Three charts measuring at three planes on a taper, differing by up to 0.8 mm. Every size page publishes the whole span and says why it is wide. |
Sources disagree: BSPT male thread diameter, every size.
All three are correct measurements of the same fitting. A 1 in 16 taper loses 1.59 mm of diameter over 25.4 mm of length, so where the caliper sits decides the reading. The size page publishes the whole span, and the basic column carries the gauge-plane diameter from ISO 7-1 Table 1, which is the figure a thread gauge checks.