First a comment on single lead vs. double, triple, or even quad or five start threads. I have seen beautifully made pens with all of these. One of my favorite "maker" pens is a Micarta (fabric in phenolic resin) pen from Skogsy Pens. It is a single lead thread and I love it. 1 3/4 turns and the cap is off. I have a friend that single points his threads on a vintage Southbend 9A metal lathe and does quad start 32 pitch threads. They are magnificent. My point is that you can do what you want. If your pens are well made and designed, people will want and love them.
My 2 cents on cap/barrel thread size...
- I started with M13.
- I also make pens with M14.
- I have M12 and M15 but haven't designed pens around them just yet.
- If you want to, at some point use a #8 size nib, an M13 thread may be crowding things and making some walls pretty thin.
- Beyond the aesthetic, a pen needs to designed to be comfortable to hold and write with for an extended period. You can make a beautiful pen, but if it isn't comfortable to use, it will be set aside for something that is. Why is this an issue with thread size? Because the thread size and the outer diameter of the barrel dictate the "step" between them. Too large a step in the wrong place can make a pen uncomfortable to hold. So... If you want to make big pens, you will probably want big threads.
I hope this all makes sense...
No matter what, the most important thing is to enjoy the journey!
David