If your internet search brought you here because you ran into the same problem I did, then welcome. I bought two of these nice little pens a while ago:
They are small, so they fit in a pants pocket, and the inky part is securely covered by a snap-on cap, so you don't have to buy new pants as often as you might with a clicker-type pen (where you are only one accidental click away from having a bad day). When using the pen, the cap fits on the other side of the pen, thus giving it some additional length. But the cap is held on to the other side of the pen not with the aforementioned reassuring snap, but via friction with an o-ring:
With an extremely limited statistical sample (there were two pens in the package), I can report that 50% of the pens continue to function well after a few years, while the other 50% do not effectively retain the cap when in use. The o-ring was too thin to start, or it has shrunk, dried up, or become frictionless in its old age, but any case it doesn't work, and the cap is irritatingly loose when writing with the pen.
Here are things that didn't work for me:
- See if Zebra Pen customer support could help. They answered promptly, told me they didn't sell replacement parts, but if I mailed in the defective pen, they would send me a replacement. Economic analysis made this a non-starter. (Cost of pen: slightly less than $3. Cost of postage: slightly more than $3.)
- Seek replacement o-ring locally. My favorite hardware store had a wide variety of o-rings, but they were mainly for plumbing and none of them matched the one from the pen.
- Seek clever solution from the internet. The o-ring from a Maglite Solitaire flashlight was reported to be a good replacement. Yay. Cost of Maglite Solitaire o-ring (assuming one didn't already have one): $3.30 from Maglite, plus $6.68 shipping. Boo.
It is ironic that the Zebra Pen slogan is, "Find zen in your pen," because the solution was inspired by the memorable scene in Zen and the Art of Motocycle Maintenance where the author offers a shim cut from a beer can as an ideal fix for slipping handlebars.
In this case, the enlightened solution was effected by removing the o-ring, wrapping a single layer of dental floss in the gap, and then reinstalling the o-ring, now with an outer diameter enlarged just enough to provide the desired friction.
May you find peace through the proper use of shims.