found this online in another fourm: ""Whitworth" refers to several schemes" "BSW"= British Standard Whitworth. This is a coarse thread equivalent to SAE, but not interchangable because BSW takes a 55 degree thread profile rather thatn the american 60 degree. Used mostly for steel into aluminum and low precison applications like case screws and through bolts. "BSF" =British Standard Fine This is fine thread like SAE fine. Used mostly for steel to steel precision and high strength parts like rod bolts. "BSP" = British Standard Pipe Just like american standard pipe this is a tapered thread, used on bikes mostly for fuel and oil fittings. NB. The size of a Whitworth wrench is the size of the nut or bolt head whch matches that bold or stud size. For instance, a 1/4" Whitworth wrench is NOT 1/4" across the jaws or the hex. Rather, it is the size of NUT that fits on a 1/4" BSW or BSF bolt. This is the same size as the hex head on that same bolt. Ditto for all of the other sizes: A 1/2" Whitworth wrench fits on the head of or on the nut that fits a 1/2" BSW or BSF bold. The same hex sizes are used for tapered fuel fittings where the conical fitting is hald together by a com- pression nut that thightens down on a BSP thread spigot."