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British Standard Whitworth (BSW) &
British Standard Fine (BSF)
screw thread series

These series are obsolete but in current production.

The pitch is the reciprocal of the number of threads per inch. The depth is 0.64033 × the pitch. The tap drills suggested below are for, roughly, a full thread.

outline of Whitworth thread form

Diameter
inches
BSW BSF
Threads
per inch
Tap
drill
Threads
per inch
Tap
drill
¹⁄16 60 57
332 48 50
¹⁄8 40 40  —
532 32 31
316 24 28 32  
732 24 17 28  
¼ 20 9 26 4
932 26 C
516 18 ¼ 22 G
38 16 516 20 P
716 14 T 18 38
½ 12 Z 16 716
916 12 1532 16 ½
58 11 1732 14 14 mm
1116 11 1932 14 39/64
¾ 10 4164 12 17 mm
1316 10 4564 12 2332
78 9 ¾ 11 20 mm
1516 9 1316
1 8 5564 10 23 mm
1 1/8 7 63/64 9 26 mm
7 1 7/64 9 29 mm
1 3/8 6 1 11/32 8  
6 1 9/16 8 35mm
1 5/8 5 1 25/32 8  
5   8  
1 7/8 4.5   7  
2 4.5   7  
2 1/8 4.5      
4   6  
2 3/8 4      
4   6  
2 5/8 4      
3.5   6  
2 7/8 3.5      
3 3.5   5  
3.25      
3.25      
3      
4 3      
2.875   Never defined.
5 2.75  
2.625  
6 2.5  

There existed as well a separate set of BSW form threads used only on bicycles (these are distinct from the BSC or CEI series):

Major diameter
inches
Threads
per
inch
7/16 20
½ 20
9/16 20
5/8 20
11/16 20
¾ 20

sources

Dimensions of Whitworth’s original proposals.

British Standard 95-1919. Corrections to Effective Diameter required to compensate Pitch and Angle Errors in Screw Threads of Whitworth Form, Tables of.

British Standard 190-1924. British Standard Whitworth (B.S.W.) Bright Hexagon Bolts, Nuts, Set-Screws, Split-pins, Washers and Studs, Dimensions. Addendums in June 1928 and September 1932.

British Standard 193-1929. British Standard Whitworth Small Hexagon (B.S.W.S.) Bright Hexagon Bolts, Nuts, Set-Screws, Split-pins, Washers and Studs. Addendum Sept. 1932.

British Standard 84-1940. Screw Threads of Whitworth Form. (Incorporating B.S. 92).

British Standard 84:1956.

resources

In the United States, BS Whitworth cap screws, hex head bolts, nuts, socket wrenches, including impact sockets, taps and dies and thread gauges are available from www.britishfasteners.com

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