Have you ever found it difficult to install a new o-ring at the end of a snorkel or touchdown barrel? Well, I certainly did.
Mr. Binder suggests using a specially fabricated tool (or a wooden dowel of the right diameter) to help.
Lacking either tools, I've adopted a humble solution to the same problem, one requiring only the use of a few paperclips.
I've found that it's very easy to push a length of a few millimeters of the o-ring into its groove. Once it's there, keep it there with a slightly sprung paperclip, push another piece of the o-ring into its recess and clamp it there with another paperclip. You'll find now that it's very easy to finish the job.
I've done two pens today with this method, a snorkel and a touchdown, with ease, while in the past it always took me a lot of trial and error to succeed.
Now I only need to find an easy method of removing an old, stubborn, petrified sac from its metal guard