Tapping and dying screw threads
Threads:
This subject has been covered in depth in past technical features, but a brief mention of the types found on most British and imported classic motorcycles may serve as a reminder.
In an ideal world, past and present motorcycle factories would have stuck to one or two thread ranges and the restorer’s lot would be far easier. But they didn’t, and in certain cases still don’t. Therefore, when working on a project, save and label every fastener removed. They will serve as patterns and to identify thread pitches and diameters when progressing the rebuild. Threads apart, these saved items will confirm hexagon sizes (these often didn’t follow form either) and the material used in the manufacture of the original fastener.
British Standard Cycle (BSC) thread:
Identical thread form to Cycle Engineers’ Institute (CEI) thread which, as far as we are concerned, it replaced. A glance at BSC and CEI tables illustrates odd threads per inch (TPI) variations between some unusual sizes listed, nonetheless the 60 degree thread form remains identical. CEI was formulated in 1902 before which cycle and motorcycle makers in effect made it up – these efforts often known in the trade as cycle threads.
Cycle threads have been used extensively in the cycle and motorcycle trade since, by makers for whom ‘Imperial’ threads were the choice. Cycle threads by nature are of fine pitch and as such have two major advantages over courser threads (such as BSW, UNC or coarse metric threads) as they
