British Standard Whitworth
The nuts and bolts on the TC are BSW, except that the bolts on the XPAG engine are metric, but with BSW size bolt heads and nuts. So I bought a set of BSW combination spanners and a set of BSW sockets.
However, the sizes are quite confusing. For example, my 3/8" BSW spanner (and socket) fits an 11/16" AF bolt head, but on the TC the bolt shank diameter is 7/16"! It seems that the size of a BSW bolt is defined in terms of the diameter of the shank of the bolt (and therefore of the thread).
One source of information is the University of Dundee, which contains the following:
| Bolt Diameter | Bolt Head AF |
|---|---|
| 3/16" | 7/16" |
| 1/4" | 1/2" |
| 5/16" | 19/32" |
| 3/8" | 11/16" |
| 7/16" | 13/16" |
| 1/2" | 29/32" |
| 9/16" | 1" |
This table (taken from the TVS Fasteners web site) shows threads per inch for a number of thread types, including BSW. Note that UNC and BSW are very simmilar. However, Withworth threads have an angle of 55° at each peak and valley, whereas UNC has an angle of 60°. Also, the peaks are rounded on BSW threads and cut off on UNC threads.
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© David James 2016 Last updated: 24th October 2016