In many vintage fountain pens celluloid tend to shrink with the age, and the rings of the cap become loose.
I'm not interested in solutions like using a drop of glue, and I machined a tool a year ago to tighten rings, but I'm not happy with it. It works, but only for very thin rings.
On the other hand, a couple of years ago I sent back to Stipula a Nettuno pen, not that old as I think I bought it 20/25 years ago, because of this problem, and the pen came back to me with rings perfectly tightened to the cap. For sure Stipula has no spare parts for this old pen, therefore it is possible to do this, and also with very good results.
I've not found any idea about how to do that on the web: anybody has an idea of what kind of tool can do that ?