Dan, something thing to remember, while a multi lead thread generally takes less turns to travel it's distance it also has three or in some cases 4 possible places to engage the threads.
This really is of no consequence unless you are making caps that post, try it with a Baron, on a roller ball no problem, but with a fountain pen where will the clip be? could be at any of three or four locations, one is great;hopefully it will line up with the nib.
Now consider making a snap cap pen, it makes no difference, and most of the top pens are snap cap, True you have to think things through a bit, bu remember acrylics and other PR materials do have a bit of flex, so the can expand to fit over a small ridge,
If you just want to make a simple fountain pen, there really is no need for the 200.00 set of taps that were ordered with the wrong relief, all you need is a tap and die for the front section I believe it was a 10 x.75 mm which is available from several machine shop supply company's.
I may be mistaken about the pitch of the thread, but that's easy to find out just ask Texatdurango, also there are some very fine taps and dies in 1/2 inch sizes if you simply must do screw caps, you can buy 1/2 56 tpi. tap and die for about 45.00 for both, the only potential down side is you may have to turn the cap 6 to 8 revolutions to get it off, or thread mill to a very coarse thread like 1/2 in by 1 or 1-1/2 tpi. for wood and threading other than the very coarse tpi. will give headaches, Ebonite, Catlinite, Bakelite, most PR will be better.
