Hi Robert,
I've made a few different style pens with the threads on the grip-section. One was a bulb filler from soligen's tutorial he posted a while back. The other incorporates the same grip-section, but in a different style pen. The third pen places the threads on the very front of the grip-section.
Left hand threads... But to address the other part of your question, I don't use left-handed threads though. Instead, I use a rubber o-ring. The o-ring keeps the smaller threads tight so they don't loosen. The o-ring adds an additional feature in that it creates a seals allowing the barrel to be filled directly with ink.
Weak joint... I've not had any issues with weakness in any of the parts when doubling up on the threads. But in my case, I use a larger set of (custom sized) threads for the outside. And on the Nanga, I taper the barrel enough that the cap can't be overtightened.
Here are a few links to the pens I mentioned:
1. This link is to the bulb filler's I made using soligen's tutorial (with the threads on the grip-section):
JEB's Pens - Custom Fountain / Bulb Fillers
2. This is s small pocket fountain pen I make (call the "PFP") that incorporates the same double threaded grip-section:
JEB's Pens - Custom Fountain Pens / PFP.
3. This pen is different from the other two. On this one, I placed the threads on the very front of the grip-section (up at the nib):
http://www.jebspens.com/fp_custom/nanga.htm.
Hope this helps...John
John E. Brady (JEB)
JEB's PENs
www.jebspens.com
Handturned Pens/Pencils
& Custom Fountain Pens