BSP Fittings: Identification and Logic

Ordering and using BSP fittings can be confusing because the nominal sizes have no relation to thread size. They are also much less common (outside the UK) than other threads, so many people are unfamiliar with them. Fortunately, BSP sizes are easy to identify.

Background

BSP stands for British Standard Pipe. It is sometimes called BSPP (Parallel) to differentiate it from the even less common BSPT (Tapered thread). Like most other plumbing fittings, BSP threads are straight or parallel, not tapered. The outside diameter of the male thread is the same from the base to the tip.

BSP fitting sizes refer to the nominal ID of a standard pipe. This is where the confusion comes in. Most plumbing fittings are named for the thread size, but BSP sizes are not. A BSP fitting with a 3/8" OD thread is actually a 1/8 BSP size, not 3/8 BSP. Use the chart below to identify BSP fitting sizes.

Sealing

BSP fittings may seal at a concave flare at the end of the male fitting, or at the port face. Female fittings which seal at the flare must be a 2-piece design with a swivelling nut. This allows you to tighten the nut without damaging the flare. One-piece female fittings (without a swivelling nut) must use a sealing washer at the base of the male fitting. Male fittings without a concave flare also require a sealing washer. In any case, BSP fittings are designed to be used dry, with no thread sealant.

A specialized sealing washer known as a Dowty Seal (Pegasus Part No. 3240-Size) has been designed specifically for sealing BSP fittings. These washers are made of steel with a rubber insert bonded to the inside. They are thicker than Stat-O-Seals, and they are sized to match BSP thread sizes. Note that some female BSP ports are chamfered to accept a plain rubber O-ring instead of a Dowty seal. Dowty seals should only be used on ports without a chamfer, while O-rings should only be used on chamfered ports.