British Standard Fine

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British Standard Fine (BSF) is a screw thread form, as a fine-pitch alternative to British Standard Whitworth (BSW) thread. It was used for steel bolts and nuts on much British machinery, including cars, prior to adoption of Unified, and later Metric, standards. For highly stressed conditions, especially in motorcycles, a finer thread, British Standard Cycle (BSC), was used as well.


BSF was developed by R. E. B. Crompton, and his assistant George Field. [1] BSF threads use the 55 degree Whitworth thread form. It was introduced by the British Engineering Standards Association in 1908. [2]

The table provides BSF sizes, the threads per inch and spanner jaw sizes. The BSC column indicates where BSF and BSC threads match. The table shows suitable tapping drill sizes. Uncommon sizes are shown in italics.


per inch/TPI)
As BSC?NutTap drill
3160.194.8332Yes8.591132 6pt532 in
7320.225.5928No??4.6 mm
140.256.3526Yes11.30--5.3 mm
5160.317.8722No13.3517321764 in
380.3759.532015.24588.2 mm
7160.4411.181818.0311169.7 mm
120.5012.701620.831316716 in
9160.5614.221623.37151612 in
580.6316.001425.651 6pt14 mm
11160.6917.531427.941 18 6pt15.5 mm
340.7519.051230.481 14 6pt16.75 mm
780.8822.351133.021 5162532 in
11.0025.401037.591 1222.75 mm
1 181.1328.70942.421 111626.50 mm
1 141.2531.75947.241 7828.75 mm
1 121.5038.10856.392 1434.50 mm

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  1. Glanfield, John (2001). The Devil's Chariots. Sutton. p. 87. ISBN   0-7509-4152-9.CS1 maint: ref=harv (link)
  2. Sidders 1969, p.16