ISO 228-1 · BS 2779
A 55 degree Whitworth parallel thread that cannot seal on itself - the joint seals on a bonded washer, an O-ring or a cone at the face.
12 sizes55 degree flanksParallel threadParallel thread, sealed on a face gasket
A straight thread with a flat shoulder or a chamfered face behind it, carrying a bonded seal washer or an O-ring.
Where it seals: A bonded seal washer or O-ring at the face. The parallel thread cannot seal on itself.
A parallel thread cannot seal on itself. If the bonded washer or O-ring is missing, the joint will leak however tight it is, and tightening it further only strips the port.
Sized by trade size like NPT, and like NPT the number is not the thread diameter: G 1/4 measures about 13.16 mm across the threads.
Where you will find it: European, British and Asian hydraulics and pneumatics. Very common on imported equipment and on almost all air compressors outside North America.
Also searched as: BSPP, G thread, BSP parallel, BS 2779.
All sizes
| Size | Thread | TPI / pitch | Basic major Ø | Measured across crests | Tube / trade size | Working pressure |
|---|---|---|---|---|---|---|
| G 1/16 | G 1/16 (28 TPI, 55 degree) | 28 TPI (0.907 mm pitch) | 7.72 mm / 0.304 in | Not published | 1/16 in trade size | Manufacturer-specific |
| G 1/8 | G 1/8 (28 TPI, 55 degree) | 28 TPI (0.907 mm pitch) | 9.73 mm / 0.383 in | 9.6 mm to 9.7 mm | 1/8 in trade size | Manufacturer-specific |
| G 1/4 | G 1/4 (19 TPI, 55 degree) | 19 TPI (1.337 mm pitch) | 13.16 mm / 0.518 in | 13 mm to 13.2 mm | 1/4 in trade size | Manufacturer-specific |
| G 3/8 | G 3/8 (19 TPI, 55 degree) | 19 TPI (1.337 mm pitch) | 16.66 mm / 0.656 in | 16.5 mm to 16.7 mm | 3/8 in trade size | Manufacturer-specific |
| G 1/2 | G 1/2 (14 TPI, 55 degree) | 14 TPI (1.814 mm pitch) | 20.95 mm / 0.825 in | 20.8 mm to 20.9 mm | 1/2 in trade size | Manufacturer-specific |
| G 5/8 | G 5/8 (14 TPI, 55 degree) | 14 TPI (1.814 mm pitch) | 22.91 mm / 0.902 in | 22.8 mm to 22.9 mm | 5/8 in trade size | Manufacturer-specific |
| G 3/4 | G 3/4 (14 TPI, 55 degree) | 14 TPI (1.814 mm pitch) | 26.44 mm / 1.041 in | 26.3 mm to 26.4 mm | 3/4 in trade size | Manufacturer-specific |
| G 7/8 | G 7/8 (14 TPI, 55 degree) | 14 TPI (1.814 mm pitch) | 30.2 mm / 1.189 in | Not published | 7/8 in trade size | Manufacturer-specific |
| G 1 | G 1 (11 TPI, 55 degree) | 11 TPI (2.309 mm pitch) | 33.25 mm / 1.309 in | 33.1 mm to 33.2 mm | 1 in trade size | Manufacturer-specific |
| G 1 1/4 | G 1 1/4 (11 TPI, 55 degree) | 11 TPI (2.309 mm pitch) | 41.91 mm / 1.65 in | 41.8 mm to 41.9 mm | 1 1/4 in trade size | Manufacturer-specific |
| G 1 1/2 | G 1 1/2 (11 TPI, 55 degree) | 11 TPI (2.309 mm pitch) | 47.8 mm / 1.882 in | 47.7 mm to 47.8 mm | 1 1/2 in trade size | Manufacturer-specific |
| G 2 | G 2 (11 TPI, 55 degree) | 11 TPI (2.309 mm pitch) | 59.61 mm / 2.347 in | 59.5 mm to 59.6 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 228-1, BS 2779. 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 7 independent published sources:
| Column | Sources | What they gave |
|---|---|---|
| Basic major diameter | iso-228-1, amesweb-bspp, machiningdoctor-bsp | Major, pitch and minor diameters read directly from ISO 228-1:2000 Table 1, and matched to three decimal places by two independent reproductions. |
| Measured male thread diameter | gerrard-id, adaptall-british, gates-idchart, apex-threadid | Four identification charts. Three print identical figures at every size; the fourth rounds up by 0.1 mm. The span covers all four. |
| Working pressure | gates-idchart, mcgill-id | Neither identification source gives a working pressure for a BSPP connection, and none is published. It depends on the port form and the seal, not on the thread. |