If you're talking about a T-mount thread, like for a 35mm camera, then its usually an M42 x .75
You can calculate the major, minor and pitch diameters using formulas in the "Metric Screw Threads - M Profile" section of the Machinery's Handbook (p. 1531 in my 23rd ed.). There are also tables to calculate your tolerances based on what grade of accuracy you require, length of engagement, etc.
If it is indeed for a 35mm camera then I would guess that you don't need a very fine tolerance grade, since concentricity doesn't usually matter much when you're just positioning an image onto film (or a CCD). Light leakage is not a problem either, since light is extremely easy to seal out (it can't easily go around a corner). It doesn't need to be liquid tight.
If it is for an application where the light image passes on to another lens, then you will need to choose a tolerance grade that ensures sufficient concentricity between the lenses.
Yes it is for a T mount I don't a machinest handbook that is one reason I'm on here. I want to make a adaptor amd need to know the data for a M42 .75 thread so I can turn one it for a inside thread.
Without knowing what tolerance grade the mating part was made to, it's tough to know exactly, but assuming it's for a film application, I'm guessing a G7 fit would work.
So the dimensions would be as follows:
M42 x .75
Major dia. = 42.02/42.30
Pitch dia. = 41.53/41.70
Minor dia. = 41.21/41.44
