Older cars and bikes,made before the 60's often use fixings that need imperial or inch based tools.Whitworth,BSF,BA or AF spanners are all called imperial and are the tools used on these early vehicles allthough some much later specialised vehicles carried on using whitworth for years after others had stopped..Many vehicles made up to the mid 70's use bolts that need AF spanners or sockets and later ones use mainly metric.There are no dates set in stone,but the older the vehicle the more chance your metric tools won't fit the majority of the nuts and bolts that hold it together.So,how do you tell the difference?.
First the easy bit.Metric tools fit metric threaded nuts and bolts.the odd one might fit a whitworth,BSF,BA or UNF nut or bolt but it wasn't designed to.If it says 19mm on the spanner it's made to fit a nut or bolt 19mm across the flats (ie) wide.Thats it,simple and really rather boring.Metric measurements are gradually being forced on the UK, and modern vehicles and machinery use metric fastenings,so little manufacturing takes place in the UK now that most of what you buy is made abroad anyway.
Older machinery,especially that made in the days when the UK had a manufacturing industry and exported rather than simply importing,will often have fixtures and fittings using imperial measurements.
Remember the term "across flats" ?. If you find a spanner marked say 1/2 AF,it means the spanner is designed to fit a fastening 1/2 inch wide or across flats (ie) a.f. The spanner might simply be marked 1/2 or 1/2 SAE some are even marked something like .50 SAE (the decimal equivalent of 1/2 inch) for example.The thread of the bolt should be either UNF or UNC but the spanner carries the head sizes stamped on it,all simple easy stuff.
With British Standard Whitworth (Whit) or BSF the spanner or socket might carry the markings for two sizes for example 7/16 inch whitworth and 1/2 inch BSF.This means the tool will fit nuts and bolts with those thread sizes.Sometimes only one standard will be marked on the spanner but a 7/16 whit will always fit a 1/2 bsf and all through the range the spanners are will fit both standards always one size apart so you only need one set of spanners to do both! British Standard Whitworth incidentally was one of the first attempts at a standard or universal thread and is named after the British engineer who invented it,BSF stands for "British Standard Fine" it is in effect the fine version of Whitworth.Thread guages which look a bit like a miniature saw blade are the easiest way to identify the different threads,a lot of bolts are marked but it's almost impossible to list all the markings that have been used by different manufacturers over the years.A spanner or socket should fit snugly on a nut or bolt in good clean condition,if it's loose its probably the wrong one.
The other 'imperial' size to confuse things is BA. BA fittings are used on lots of older machinery especially electricl stuff,the common sizes are 4 and 2 BA.The smaller the number the bigger the spanner and they go from 0 to 11BA.All BA fixings have a set thread and head size so a 2BA spanner fits a 2BA socket or bolt.We sell a big variety of imperial based tools so the people who tell you whitworth,BA and af are obsolete or expensive are wrong,we sometimes list some of these tools on ebay,all at 99p reserve.
Some nuts and bolts although of different thread forms and sizes are similar enough that they can be swapped over although seeming to be either lose or tight,for example some whitworth and UNC fittings can be similar enough to screw onto one another.This practice is wrong and in certain circumstances like highly stressed fittings on brakes or suspension components can be dangerous.Once again using a thread gauge will avoid using incorrect fittings and mixing incomparable imperial,or mixing imperial and metric fixings.
Complicated? Maybe but all part of the fun of owning a vehicle or machinery made before the days when plastic and planned obsolescence became the norm.Best of luck with it.