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

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These series are obsolete.

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
116 60 57
332 48 50
18 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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