Mystery socket sizes?

[Richard Farrant]

Originally Posted by motto

As I understand it the British marked their spanner/socket sizes based on the shank/thread diameter of the bolt, not the distance across the flats (A/F)
I believe that the distance across the flats was arrived at by applying a ratio to the bolt/shank/thread diameter which usually ended up being an odd sort of in between measurement for the distance across the flats.
The Americans went the simpler route and rounded head size to the nearest 1/16"( mostly) and marked their tooling accordingly.

David

Hi David,
I think that was correct pre-WW2, but to save metal the hex head and nut was reduced, so a 1/4" BSF or BS Whit bolt needs a 3/16" Whit spanner, 5/16" bolt needs a 1/4" Whit spanner and so one.

To go slightly off course, I get truck drivers coming into my workshop asking if I have a 10mm bolt, when I question them, it turns out they want a 6mm bolt, they go by the spanner size. Same used to happen with Unified and BS.

regards, Richard