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BOULONNERIE
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Thread sizes and head sizes on British bikes.
Before 1968 the motorcycle industry used the British Whitworth based sizes like BSF and CEI or Cycle or BSC (26 Threads per inch). The size on the spanner indicates the size of the thread/diameter of the bolt. The number is always followed by W or WW or BSW or BSF.
In 1968 they started changing to American sizes. AF (across flats) the size on the spanner is the size of the head. The size on the spanner is just a number.
These systems don’t interchange.

Check your bolts and  with a thread gauge and choose:

CEI/CYCLE THREAD

UNC

UNF