ES member LightningRods found many inconsistencies in the standard BSA bottom-bracket thrreads, It was sometimes listed as 1.38 X 24, 1.375 X 24, or 1.37 X 24. Flanged freewheels (ENO, ACS, etc) and BB components are now sometimes used for jackshafts in non-hub drives.
BSA stands for "Birmingham Small Arms", a military firearms company in England that established many machining standards in the early days of the mechanization of industry.
Here is the discussion about finding the proper exact dimensions:
https://endless-sphere.com/forums/viewto ... =2&t=54237
1.375 X 24, ISO 6696 thread standard, 2008