British Standard Fine

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.

Table

Diameter			Thread density (threads per inch/TPI)	As BSC?	Nut		Tap drill
(inches)		(mm)	(in^−1)		(mm)	Approx. (inches)
​^3⁄16	0.19	4.83	32	Yes	8.59	^11⁄32 6pt	​^5⁄32 in
​^7⁄32	0.22	5.59	28	No	?	?	4.6 mm
​^1⁄4	0.25	6.35	26	Yes	11.30	--	5.3 mm
​^5⁄16	0.31	7.87	22	No	13.35	^17⁄32	​^17⁄64 in
​^3⁄8	0.375	9.53	20		15.24	^5⁄8	8.2 mm
​^7⁄16	0.44	11.18	18		18.03	^11⁄16	9.7 mm
​^1⁄2	0.50	12.70	16		20.83	^13⁄16	​^7⁄16 in
​^9⁄16	0.56	14.22	16		23.37	^15⁄16	​^1⁄2 in
​^5⁄8	0.63	16.00	14		25.65	1 6pt	14 mm
​^11⁄16	0.69	17.53	14		27.94	1 ^1⁄8 6pt	15.5 mm
​^3⁄4	0.75	19.05	12		30.48	1 ^1⁄4 6pt	16.75 mm
​^7⁄8	0.88	22.35	11		33.02	1 ^5⁄16	​^25⁄32 in
1	1.00	25.40	10		37.59	1 ^1⁄2	22.75 mm
​1 ^1⁄8	1.13	28.70	9		42.42	1 ^11⁄16	26.50 mm
​1 ^1⁄4	1.25	31.75	9		47.24	1 ^7⁄8	28.75 mm
​1 ^1⁄2	1.50	38.10	8		56.39	2 ^1⁄4	34.50 mm

References

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

Bibliography

• P.A. Sidders, ed. (1969). Guide to World Screw Threads. New York: Industrial Press. ISBN 0-8311-1092-9.

External links

• Thread sizes
• Jaw sizes for sockets and spanners/wrenches
• Spanner size chart