BSPP Pipe Thread (ISO 228-1 / G Series)
Direct Answer
BSPP (British Standard Pipe Parallel), designated with the prefix 'G' under ISO 228-1, is a straight cylindrical pipe thread with a 55° Whitworth rounded crest and root profile. Unlike tapered threads, BSPP threads do not seal along the thread flanks, requiring an external bonded elastomeric washer (Dowty seal), O-ring, or copper crush washer on a flat mating shoulder.
Source: ISO 228-1:2000 (BS 2779)
Specs
| Standard Governing Body | ISO 228-1 / BS 2779 / DIN ISO 228 |
| Thread Designation | G (e.g., G 1/2", G 3/4") |
| Thread Form & Flank Angle | 55° Whitworth with radiused crests and roots |
| Taper | None (0° — straight cylindrical thread) |
| Common Pitches (TPI) | G 1/8": 28 TPI | G 1/4"-3/8": 19 TPI | G 1/2"-3/4": 14 TPI | G 1"-2": 11 TPI |
| Sealing Mechanism | Bonded seal (Dowty washer), O-ring, or elastomeric gasket on shoulder |
| Primary Geographical Region | Europe, UK, Australia, Asia, and global industrial hydraulics |
55° Whitworth Form and Parallel Cylindrical Profile
BSPP threads utilize the classic $55^\circ$ Whitworth thread profile where both thread crests and roots are rounded off to one-sixth of the fundamental triangle height ($r = 0.137329 \times P$). Because the male and female diameters remain constant along the entire length of engagement, the joint can be threaded together freely without wedging. This makes BSPP ideal for hydraulic manifolds, water meters, and pressure transducers where predictable positioning and vibration resistance are required.
Shoulder Gasket Sealing vs Thread Sealing
Because BSPP threads are parallel, applying PTFE tape to the threads does not create a reliable seal and may crack female ports. Sealing is achieved exclusively at the interface between the male hex flange and the female counterbore using:
- Bonded Dowty Washers: Metal retaining outer ring bonded to a nitrile or Viton inner elastomeric bead.
- Elastomeric O-Rings: Compressed into a precision machined chamfer at the base of the male thread.
- Soft Copper or Aluminum Washers: Plastic deformation crush seals for high-temperature fluid lines.