Did BSA not have torque wrenches?
Oh I don't know but probably they did, they also built armaments which would be up to and tested to spec from ministry men.
However Bert on the assembly line would have a good idea where tight enough was after the first couple of hundred heads he had done.
and at home the average rider would be fairly safe with a standard length set of spanners unless he was called Dan and ate cow pies, trouble starts with that bit of pipe slipped over the end of the spanner