Modern metric spanners are marked or sized in the same way as AF but in millimetres,the distance again between parallel faces of an open ended spanner or wrench.They are used on metric threaded fastenings,these have a thread angle of 60 degrees. Metric bolts often have two numbers on the heads,this is a strength rating. Coarse threads (whitworth,unc etc) are more likely to be used on the thread in a casting(especially aluminium) to give greater strength,most nut and bolt assemblies will use a fine thread,so studs in castings often have a coarse thread end to go in the casting and a fine end to take a nut. Finer threads (BSF,unf etc) have a greater resistance to being shaken loose by vibration.Bolts and nuts in brass or similar soft metal are normally coarse threaded for greater holding power..

B.A (British Association) are smaller sized fastenings found on older vehicles electrical fittings,magnetos,dynamos,motors,pumps and instruments etc. B.A thread has an angle of 47 degrees-30 minutes.This was another standard introdued to try and rationalise the sizes of smaller fixings,and which now confuses greatly the metric spanner wielding enthusiast. Whitworth,BSF,metric and imperial spanners are based on different standards and hence sizes,with few exceptions they are not interchangeable..As well as classicmotorcyclesfrom AJS to Vincents,a lot of older cars used whitworth and BSF fastenings .Marine engines like Lister and Gardner use them as well, andwere designed tolast,so imperial tools might be regarded as obsolete by some but are still essential on such units..