I keep an eye out for military spanners and I have a number of wartime /|\ marked examples now. This week however I picked up my first RAF spanner:Despite the extensive use made of tools by the RAF throughout the war, marked examples do seem scarcer than their army equivalents. This spanner is marked ‘AM’ with a crown for the Air Ministry and dated 1940:The spanner is marked on the opposite side for the thread sizes it works with. One end is for 3/8 British Standard Whitworth (BSW) and 7/16 British Standard Fine (BSF), whilst the opposite end is marked up for 5/16 BSW and 3/8 BSF:BSW was the first standard screw thread size, developed by Joseph Whitworth in 1841 and was used across the Empire. The British Standard Fine (BSF) standard has the same thread angle as the BSW, but has a finer thread pitch and smaller thread depth. This is more like the modern “mechanical” screw and was used for fine machinery and for steel bolts.
Whitworth (spanner) markings refer to the bolt diameter rather than the distance across the flats of the hexagon (A/F) as in other standards. Confusion also arises because BSF hexagon sizes can be one size smaller than the corresponding Whitworth hexagon. This leads to instances where a spanner (wrench) marked 7/16BSF is the same size as one marked 3/8W as in this case. In both cases the spanner jaw width of 0.710 in, the width across the hexagon flat, is the same. However, in World War II the size of the Whitworth hexagon was reduced to the same size as the equivalent BSF hexagon purely to save metal during the war and they never went back to the old sizes afterwards. Despite being long superseded by metric threads and bolts, these imperial threads and bolt heads are still in widespread use throughout the world, a legacy from the nineteenth century that shows no sign of disappearing anytime soon.