There is an old thread where Stuart insists that the threads in the case of a 60s unit Triumph are in fact 55 degree 1/4 BSF rather than 60 degree cycle. This is because cycle was only used into steel. BSF was used into aluminium. As the threads get larger the tpi drops with BSF whilst the big steel threads on a Triumph remain 26 tpi. apart from the big rear spindle which is 20TPI. I wonder if it is the same for a unit BSA?
Yes, in parts books for the Unit Single rocker box studs pre UNF/UNC you see the 1/4 stud is threaded BSF one end which screws into the alloy head and Cycle on the end a steel nut is used. You only know which end is which by looking at the length of the thread. The same stud ends up UNC/UNF on later bikes, showing they finally realised 26 TPI in alloy is not a good idea.