British threads include several families of thread types, including BSP, BA, and Whitworth threads. BSP stands for British Standard Pipe, and it is a family of standard screw thread types. Mainly used for interconnecting and sealing pipe ends, BSP threads are used around the world (excluding the USA). Pipe ends cut in BSP threads join by mating an external with an internal thread. The Whitworth and BA types are mainly used in machinery and precision instrument applications. BSP and other British threads and thread plug gages come in a variety of types.
BSPP stands for British Standard Pipe Parallel, and as the name implies, it uses parallel threading, which means each screw has a constant diameter. BSPP typically forms a seal with a bonded seal ring that fits between the shoulder of the male piece and the face of the female piece. BSPP thread plug gages are used to check the internal threads of BSPP joints, usually found on applications with low working pressure.
BSPT stands for British Standard Pipe Taper, and this type of screw thread features a diameter that increases or decreases along the length of the thread. The angle between the flanks is 55 degrees, measured in an axial plane. Although it does not require a bonded seal ring like the BSPP type, it does require sealant to create a complete seal. BSPT thread plug gages are used to check the internal threads of BSPT joints, which are very popular in Japan and China.
BSF stands for British Standard Fine, and this type of screw threads was invented in 1908. Prior to the adoption of Unified and Metric standards, it was widely used for steel bolts and nuts. It is a fine-pitch alternative to BSW threads (described below). BSF threads use a 55 degree Whitworth thread form and have been used in cars and other machinery. BSF thread plug gages are used to check the internal threads of BSF parts.
BSW stands for British Standard Whitworth, and this type of screw was the first national screw thread standard in 1841. Today, it is mainly used in pipes, cooling systems, and pneumatic and hydraulic systems. Along with the BSF and BSC threads, it makes up the group of Whitworth threads. BSW thread plug gages are used to check the internal threads of BSW parts.
BA stands for British Association, and BA screw threads are used for very small and precise applications. BA standard threads are recommended for screws less than ¼ inch in diameter. Although largely obsolete now, BA screw threads were one of the first to adopt the metric system, with the pitch of the smallest size being 1 mm, and are still used in optics and relays, among other applications. BA thread plug gages are used to check the internal threads of precision parts.