Bspp
BSPP (British Standard Pipe Parallel) is a 55° Whitworth-form parallel pipe thread defined under ISO 228. Unlike BSPT, the thread itself is not intended to create a pressure-tight seal; sealing is achieved by a separate washer, bonded seal, O-ring, or 30° cone seat. Sizes are designated G 1/8, G 1/4, etc.
On the shop floor, BSPP appears in ported housings, hydraulic manifolds, adapters, and threaded bungs that must match British fluid-power hardware. Machinists cut the parallel 55° Whitworth form with the correct pitch diameter and TPI, then finish a spotface, chamfer, or 30° cone seat depending on the seal design. The real control variables are major diameter, thread class, and sealing-face geometry, not just nominal size. A BSPP joint relies on an O-ring face seal, bonded washer, or flat seat, so the machined sealing land must stay flat and smooth. If the port geometry is off, the fitting may screw together yet leak under pressure. Confirming the G designation and comparing thread data against the BSPP chart prevents costly rework. BSPP also appears on compressed air, coolant, and water service hardware where imported equipment requires British-threaded adapters rather than NPT or metric pipe threads.
How is BSPP identified on a drawing or fitting?
The thread is checked for parallelism, TPI is verified, and the outside diameter or female ID is measured with calipers; then it is matched to a BSPP chart. In ISO notation, BSPP external threads carry a G designation, and the fitting seals via a washer, O-ring, or cone seat rather than the thread.
What thread form does BSPP use and does it seal on the threads?
BSPP uses the Whitworth 55° thread form. Under ISO 228, pressure-tight joints are not made on the threads; sealing is achieved with a separate element such as an O-ring, bonded washer, or 30° cone seat.
What machining features matter most on a BSPP port?
The sealing face usually controls leakage: spotface flatness, cone angle, O-ring gland depth/width, and surface finish matter more than thread depth alone. Correct major diameter, pitch, and thread class are also required to match British fluid-power hardware.