Lennart, The dimensions that MD gave you, In looking at the Machinists book thread is 5/16-32 UNEF (Unified National Extra Fine). In the machinists book it lists the external (bushing) major diameter as .3115 and minimum at .3055. Internal (hole) minor diameter minimum is .279 and max is .286 and major diameter is .3125 (exact size of 5/16).
I pretty sure that L.C. Smith used Unified National sizes instead of their own design threads.
David

David