Are there different bottom bracket thread diameters?
 

I'm looking to replace a BB in a custom frame I bought used, I know I need to measure the shell and spindle width, but do I also need to know the diameter for the threading or are they all the same?

What is the frame? Will likely be English or Italian Thread, on a road bike. They are different diameters, yes

Ok, thanks, it's probably from a local builder (I'm from Argentina) but it has a Cinelli shell, does that mean it's an italian thread?

It depends on when it was built. Older Cinellis would be Italian, but sometime in the eighties (± a decade or so) they went to ISO or British threading. If you have a good eye one easy to tell is by the thread direction of the right cup. RH thread = Italian, LH thread = ISO. Otherwise you can use a vernier caliper, ISO ID is slightly under 34mm, and Italian ID slightly under 35mm.

Do you have a friendly bike shop nearby ? Maybe they have some old cups laying around with the threading stamped on them, and you can see which ones fit. If the threads are clean, you should be able to thread some in by hand, or not, without damaging the frame. Don't use tools to force it.

Thank you, I appreciate the help :)

No. Cinelli provided shells in English, Italian, and metric ("French") thread. You'll need to measure, or at least try a cup of known thread in the shell to see if it fits.

N.B. metric thread Cinelli shells are fairly uncommon, and can be distinguished by the marking "S. C. France" on the bottom of the shell:

http://www.os2.dhs.org/~john/cinelli-france.JPG

Are there markings on the existing BB cups? Here's a translation:

1.37 x 24t = English thread
36 x 24 = Italian thread
35 x 1 = French thread

Good luck!