The British threads family includes BSP, Whitworth, and BA thread types. Although all have similarities, they serve different applications. The BSP threads are used for pipes, and the Whitworth and BA threads are used in machinery and precision instruments. All the external threads in any variety of British screw threads need to be tested for standards compliance with ring gages. A ring gage is a calibrated ring with internal threads, often with GO/NOGO sides. By screwing it onto the external threads, a user can determine if the external threads comply to standards or need to be replaced. If the GO side threads on and the NOGO side does not, the part complies to standards. This calibration helps maintain consistency and safety.
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 ring gages are used to check the external 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 ring gages are used to check the external 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 ring gages are used to check the external 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 ring gages are used to check the external 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 ring gages are used to check the external threads of precision parts.