Believe it or not, but it is a standard thread or at least it was donkey’s years ago. British Standard Cycle Thread superceded the previous Cycle Engineers Institute threads, and it is a common misconception that all BSC threads are 26TPI. In fact in various sizes above half an inch diameter there is an alternative at 20TPI that was a legacy of the CEI days. I own a superb set of BSC taps and dies and amongst them are a few 20tpi items AND some left hand thread items too that I believe relate to pedal cycle cranks. The wooden case stencil markings suggest that it is an ex-government set from the 1939-1945 era.
