Every mechanic should have a thread gauge:
A lot of threads were changed on Norton motorcycles after production was moved to the AMC works, and later in the 70s just about every type of thread found it's way onto British motorcycles, SAE, whitworth and metric.
I was looking at a small screw on my 62' 650ss the other day trying to figure out what it was, it had more TPI than a 6-32 sae tap, but less than a 4-40 tap, so I looked up a table of british threads and behold there was the 3BA British Association specification that had about the same O.D. as and SAE 6-32 screw but with 34.84 threads per inch.
On Norton fuel tanks:
The Norton's from the old works usually all had their fuel caps on the left, so when the bikes were put on the side-stand with a full tank they very well could weep fuel down the side of the bike onto the engine if the cap seal was not perfect.
Most post 62' Nortons had their caps on the right side of the tank, a sensible modification, one exception being the Long Range Commando fastback, which had an old Dominator tank with modifications to make it fit onto a Commando frame, it's fuel cap for some reason was located further forward and to the left than any pre-63' Dominator! I guess it may have been out of the way of a tank bag anyway.