I cannot get my pen to take an adequate picture of this, so I will have to describe my problem.
I am working on an Osmiroid pen, probably, I think, from the 70's. It is red, modern plastic, uses the tips which interchange with Esterbrooks, has a black dome ending off the cap and fills with a lever.
The section screws in, and the cap screws on. Both sets of threads occupy the last quarter inch or so of the barrel.
There is a tiny crack in the threads that goes out to the edge, about an eighth of an inch long, and another miniscule crack that is just sitting there within the threaded area, I would never have noticed either except that it is a used pen I am fixing up to give a friend who used to have one like it years ago, and so I was cleaning it and looking carefully at the ink in the threads.
My instinct is to leave well enough alone. I can't glue anything to the back or the front to reinforce it, because there are threads on both sides, .if I try to do solvent welding (I don't know if it even works on injection molded plastic) I am afraid I might deform the threads as it would be my first try, and friend, who is not a FP person, is highly unlikely to take the thing apart and stress it that way.
Do I need, however, to do something because these situations have a known trajectory? If It were staying with me I wouldn't worry about it, but it is being sent away to live on another continent and I do not want to send a disappointment...