From: Wdcockey@aol.com
Date: Wed, 20 Dec 1995 22:12:17 -0500
To: Land-Rover-Owner@uk.stratus.com
Subject: Re: Whitworth - Info.
BA is short for British Association according to my father-in-laws 50 yr old engineering manual.

BA threads:
BA    Bolt diam.    Pitch         Head
6  2.8mm 0.110in  0.53mm 0.020in  0.193in   
4  3.6mm 0.142in  0.66mm 0.026in  0.248in
2  4.7mm 0.185in  0.81mm 0.031in  0.324in
0  6.0mm 0.236in  1.00mm 0.039in  0.412in
Head size is hex across flats and round head diameter
Above from B.S. 93:1919 and B.S. 57:1920
Note that for the bolt diameter and thread pitch the primary dimension is
metric. And that was 75 years ago.
 
BS and Whitworth hex sizes
BS   Whitworth   Hex AF
3/16   1/8        .340
1/4    3/16       .445
5/16   1/4        .525
3/8    5/16       .600
7/16   3/8        .710
1/2    7/16       .820
9/16   1/2        .920
5/8    9/16      1.010
11/16  5/8       1.100
3/4    11/16     1.200
7/8    3/4       1.300

I cant see any logical progression of head sizes but Im sure it made sense to Mr. Whitworth. I believe that the threads are very different from the more usual Unified & SAE, with different thread pitches. The Series One Club offers a Whitworth rethreading tool for straightening bolt threads.

RN & AB among others carry Whitworth/BS tools
Snap-On also has Whitworth sockets, but not wrenches

Hope the above is of help.
David Cockey