BSP

BSP stands for British Standard Pipe, a tapered thread specification that has been used in plumbing, hydraulics, and compressed air systems for over a century. The threads are cut at an angle of 55 degrees and taper at a rate of 1 in 16, meaning the diameter reduces by about 1.75mm per 25mm of thread length. This taper allows the threads to wedge together and create a seal without requiring sealant tape or compound, though these are often used anyway to ensure a watertight joint.

BSP comes in two main variants: BSPP (parallel) and BSPT (tapered). BSPP has straight threads and requires an O-ring or washer to seal; BSPT is the tapered version where the threads themselves provide the seal. Both are measured in imperial fractions of an inch, so a half-inch BSP connection is actually 21.336mm in pitch diameter. The confusing part for modern workers is that BSP measurements do not match metric sizes, making conversion charts necessary on mixed installations.

BSP threads appear on copper pipework fittings, brass compression fittings, and cast iron connectors throughout Europe and the Commonwealth. A 1/2 inch BSP male thread on a brass elbow, for example, will screw directly into a female socket of the same size. The standard is defined in BS EN ISO 228 and remains the default thread type across UK plumbing supply chains, even as metric ISO pipe threads have gained ground in some industrial applications.

Common problems and sealing

Over-tightening a tapered BSP fitting can split the female socket or strip the thread, especially on softer metals like brass; finger-tight plus one and a half turns is typical advice. Leaks often occur when the threads are damaged, corroded, or when the fitting is reused without cleaning the old PTFE tape residue. A dry joint (no sealant) on BSPT will usually weep unless threads are absolutely clean and in perfect condition.

The name reflects its origin as a standard set by the British Standards Institution in the mid-1900s. Although metric systems have displaced BSP in many industries, it remains the standard for most water supply and heating installations in the UK. When an engineer specifies "1/2 inch BSP" on a drawing, they are invoking a geometry so familiar in British plumbing that nearly every merchant will have the fitting in stock.