Hi Jack,
just for kicks and giggles I will put this up regarding Villiers from an article "The Nuts and Bolts of the Matter" .
It is lengthy but I will just put up a bit about Villiers
Villiers had a partiality for 20TPI constant pitch threading. This means that regardless of screw diameter they threaded it 20TPI. This is the same idea as the 26TPI used in British Standard Cycle thread but Villiers decided on 20TPI not the 26 of BSC. It was acceptable engineering practice of the day and in fact examples of it can be found in odd places on various British bikes. In practice this is only an issue where 5/16th x 20 thread was used as there are easily available alternatives for all the other sizes. Either BSW or UNC covers 1/4 x 20, BSF covers 3/8 x 20 and both 7/16 and 1/2 x 20 are standard UNF sizes. The only one you can't match is the 5/16 x 20. Villiers did not use it a lot so it doesn't often cause a problem.
