Spanners and Wrenches - DT Online- bosch spanner set size chart ,The spanner sizes of these, by contrast, are marked with the diameter of the bolt they would fit - e.g. a 1/2" BSW spanner will fit a BSW nut and bolt of 1/2" thread diameter. A further complication is that BSW spanners will normally also fit a BSF nut one size larger - e.g. a 1/2" BSW spanner will also fit a 9/16" diameter BSF nut and bolt.Spanner Jaw Sizes - Crawford SpaceFor the 'British' sizes, the nut/bolt size corresponds to the spanner marking (i.e. a 1/2" W spanner fits a Whitworth bolt with a shank/thread diameter of 1/2"). The original Whitworth sizes were standardised in 1841 by Sir Joseph Whitworth (1803-87) and featured a significant head oversize to accommodate the crude tolerances of the production methods of the day.