Chris, there is about 1.5mm of threaded pr under the collar. As you can see though, I made the threads long enough to go much deeper into the wider part of the body.
I wouldn't have risked it without the collar.
I was actually going to affix the nib end and then make the finial removable. In fact, the whole pen was going to be a lot fancier, it was planned to be a tri-lobe shape and with wave segments. But I really am struggling with shaky hand and fatigue. I nearly gave up completely after belting my knuckle on the chuck jaws..
Thanks for the comments guys, but I think you are being too kind...