What I would suggest is find a nib that you want to build around and work from there. At most you will need 2 sizes, one tap for the nib and one tap/die for the cap. The clip you can use the same tap/die as the cap threads.
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While I agree completely with this approach (I use a #5 Bock), I think you will need 3 sizes, not 2. You will need a tap for the feed. You will need a tap and die for the nib section to the barrel. I suggest 10mmX1.0. This tap and die can also be used for the finial to hold the clip, assuming you use a clip. You will need a tap and die for the cap to the barrel. I suggest a 12mmX.75.
The tap for the feed is an odd size and you will likely need to buy that from a pen supplier. The others are standard sizes and available from multiple suppliers on the internet. Victornet is who I've used.[/QUOTE]
I only use 2 sizes
feed size and cap size. No need for a 3rd size but that is design specific to be honest.