We are often asked for bolts or nuts by enthusiasts quoting spanner sizes, which of course don’t correlate for unified (UNC/UNF) sizes but do tend to correlate to Whitworth. If you were to look at spanners, handed down from Grandfathers or Fathers, you are likely to find those rather odd spanners known as Whitworth. often shown on the spanner as, for instance 5/16W (the W for Whitworth) and commonly used pre-war in the UK. After the war in 1948, the unified system came into being, yet Whitworth continued for many years after, possibly due to the fact that components were designed before the change.
One of the great things about Whitworth is that the nut/bolt size generally corresponds to the spanner marking in other words a 1/2″ W spanner fits a Whitworth bolt with a shank/thread diameter of 1/2″. Unlike Unified sets and bolts (UNF/UNC). Whitworth sizes featured a significant head oversize to accommodate poor production methods of that era. In the early 19th Century, the standard was revised to include the option for a finer thread and became British Standard Fine (BSF). BSW bolts use the same coarse thread as the original Whitworth, which works really well with softer materials (e.g. aluminium, cast iron). BSF bolts have the same profile but a finer cut (i.e. higher TPI value) providing better vibration resistance.
The BSF head size is one step smaller than BSW. However, during the Second World War the standards were revised as an austerity measure to reduce steel consumption and this resulted in the normal BSW head sizes being reduced by one step, basically making the BSW = BSF head size.
In 1954 a further revision occurred using ‘BS’ to refer to the BSF/BSW size and ‘W’ for the original large hexagon size. As a result, spanners are marked along the lines of ‘1/4 W 5/16 BS’ indicating the jaws are sized for a 1/4″ large hexagon Whitworth bolt, or the next step up at 5/16″ for BSF/BSW.
Typically, you will find BSF/BSW used in British equipment designed before 1948 (for instance early Land Rover and Austin Healey gearboxes).
So hang onto those “hand me down” Whitworth spanners, especially if you are working on 1950’s or 60’s machinery.