British standard fine thread

British standard fine thread (BSF) is a fine screw thread alternative to British Standard Whitworth (BSW) thread. It is used for steel bolts and nuts on much British machinery, including automobiles, prior to adoption of the metric standards. For highly stressed conditions, especially in motorcycles, a finer thread, British Standard Cycle (BSC), was used as well.

The table provides BSF sizes, the threads per inch and wrench jaw sizes. The BSC column indicates where BSF and BSC threads match.

If you need to tap new holes, and you wish to stay original BSF taps seem essential. The table also shows suitable drill sizes if matching BSF-sized drills are not available or broken. Uncommon sizes are shown in italics.

fractional inches	decimal inches	thread per inch	BSC match	Jaw mm exact	or ~inch	drill for tap
3/16	0.19	32tpi	`=	8.59mm	11/32" 6pt	5/32"
7/32	0.22	28tpi		?	?	4.6mm
1/4	0.25	26tpi	`=	11.30mm	--	5.3mm
5/16	0.31	22tpi		13.34mm	17/32"	17/64"
3/8	0.375	20tpi		15.24mm	5/8"	8.2mm
7/16	0.44	18tpi		18.03mm	--	9.7mm
1/2	0.50	16tpi		20.83mm	13/16	7/16"
9/16	0.56	16tpi		23.37mm	15/16	1/2"
5/8	0.63	14tpi		25.65mm	1" 6pt	14mm
11/16	0.69	?		27.94mm	1'1/8" 6pt	15.5mm
3/4	0.75	12tpi		30.48mm	1"1/4 6pt	16.75mm
7/8	0.88	11tpi		33.02mm	1"5/16	25/32"
1"	1.00	?		37.59mm	1"1/2	22.75mm
1⅛"	1.13	?		42.42mm	1"11/16	?
1¼"	1.25	?		47.24mm	1"7/8	?
1½"	1.50	?		56.39mm	2"1/4	?

External links

• Thread sizes
• Jaw sizes for sockets and spanners/wrenches
• imperial metric conversion charts
• Size comparisons Whitworth/BSF/BA/AF and mm
• Spanner size chart