I have several sets for three different size pens.
My smallest ones use a 1/2" x 28tpi for cap to barrel.
Section to barrel is 10mmx1, but I'm going to get a 10mmx.75, which is something like 32tpi. The finer threads cut so much easier than the coarser ones, from my experience, especially when threading PR, which most people seem not to like for kitless pens.
These are two pens made with those taps/dies. The blue has a #5 Bock nib, the dark purple a #6 Jowo nib. If you want to do full custom like this, a tap for each different nib unit is necessary. They're each $35-40 and can be found at a couple different places.
This page has many of these specialty taps listed.
silverpenparts.com - TOOLS - Taps and dies
Classic nib has them as well:
CLASSIC NIB - FEEDS
Small/Medium
The next size up uses larger threads for barrel to cap.
This one is comparable in size to a MB 146.
9/16" x 28tpi. I would never go lower than 28tpi for cap to barrel threads as I think it's just too coarse. People don't like sharp threads under their fingers. I know I don't.
It uses 11mmx.75 tap and die for section to barrel threading. Bigger pen - so more meat around the converter.
Large pen.
Last, my oversize,
MB 149 size. Or bigger...
I use 5/8" x 28tpi for cap to barrel threads, and 12mmx.75 for section to barrel. Lots of meat there. On these bigger ones I can even run a tap through the section so it will accept a Pelikan M1000 nib unit. I haven't done one of those yet, I've got so many orders to fill I can't really play around much. which is a good thing!