British Thread Data

British motorcycles used several forms of threads and it can get a bit confusing trying to identify what you have got. But if you have a vernier calliper and a thread gauge it doesn’t need to hold you up. Here are the most common ones used on most British bikes before about 1960 when pressure from the export market caused some manufacturers to start using American standards.

British Standard Whitworth (BSW)

This is the coarsest thread you are likely to find on a British bike and is normally only used for threads in aluminium.

British Standard Fine (BSF)

BSF uses the same thread form as BSW but has a finer pitch for any given diameter.

You are most likely to find these on bolts, studs, nuts and anywhere that both male and female threads are in steel.

BSF became more common after WW2 and was widely used in the 50’s and 60’s. Earlier bikes used BSCy - see below.

British Standard Cycle (BSCy)

As the name suggests this thread originates in the cycle world. Commonly used on early British motorcycles it became less common after WW2 as it got replaced by BSF.

As with BSF you are most likely to find these on bolts, studs, nuts and anywhere that both male and female threads are in steel.

Some sizes were available in 26 and 20TPI so if looking at a one of these sizes it is really important to use a thread gauge. 20TPI BSCy is probably most common on wheel spindles.

British Association (BA)

BA was the British standard for anything electrical. So on a bike we are looking at magnetos, dynamos, lights, horns and switches.

It is occasionally used elsewhere on a bike so don’t completely rule it out.

British Standard Pipe (BSP)

Again, there is a clue in the name here. BSP is likely to be used for anything connected to a pipe. So petrol and oil fittings are usually BSP, as are drain plugs and sometimes gearbox and chain case filler and level plugs.

As you can see from the table, the OD doesn’t match the description. 1/4 BSP is big enough for a 1/4 bore hole through the middle, the OD of the thread is quite a lot bigger. 1/8, 1/4 and 3/8 are the most common sizes used with a tendency for sizes to go up as the years went on - especially on oil pipes as better flow rates were demanded by engine designers.