British Standard Whitworth - Comparison of Standards

Comparison of Standards

The British Standard Fine (BSF) standard has the same thread angle as the BSW, but has a finer thread pitch and smaller thread depth. This is more like the modern "mechanical" screw and was used for fine machinery and for steel bolts.

The British Standard Cycle (BSC) standard which replaced the Cycle Engineers' Institute (CEI) standard was used on British bicycles and motorcycles. It uses a thread angle of 60° compared to the Whitworth 55° and very fine thread pitches.

(To simplify matters the term hexagon will be used in this paragraph to denote either bolt head or nut.) Whitworth spanner (wrench) markings refer to the bolt diameter rather than the distance across the flats of the hexagon (A/F) as in other standards. Confusion also arises because BSF hexagon sizes can be one size smaller than the corresponding Whitworth hexagon. This leads to instances where a spanner marked 7/16BSF is the same size as one marked 3/8W. In both cases the spanner jaw width of 0.710 in, the width across the hexagon flat, is the same. However, in World War II the size of the Whitworth hexagon was reduced to the same size as the equivalent BSF hexagon purely to save metal during the war, and they never went back to the old sizes afterwards. Thus it is today uncommon to encounter a Whitworth hexagon which takes the nominally correct spanner. Spanners in this case may be marked 7/16BS to indicate that they have a jaw size of 0.710 in and are designed to take either the (later) 7/16 BSW or 7/16 BSF hexagon.

The British Association screw thread (BA) standard is sometimes classed with the Whitworth standard fasteners because it is often found in the same machinery as the Whitworth standard. However it is actually a metric based standard that uses a 47.5° thread angle and has its own set of head sizes. BA threads have diameters of 6 mm (0BA) and smaller, and were and still are particularly used in precision machinery.

The Whitworth 55° angle remains commonly used today worldwide in form of the 15 British standard pipe threads defined in ISO 7, which are commonly used in water supply, cooling, pneumatics, and hydraulic systems. These threads are designated by a number between 1/16 and 6 that originates from the nominal internal diameter (i/d) in inches of a steel pipe for which these threads were designed. These pipe thread designations do not refer to any thread diameter.

Other threads that used the Whitworth 55° angle include Brass Threads, British Standard Conduit (BSCon), Model Engineers (ME), and British Standard Copper (BSCopper).