NEVER-USED RUBBER SACS

I don't repair many pens, and when I do, the work is mostly limited to flushing out the pen and replacing the rubber -- O rings, sacs or vac diaphragms. I have a number of these parts sitting around, and I've started to wonder if they are getting over the hill and should be replaced. How old is too old for rubber replacement parts before you should throw them out -- five years, 10 years, longer?

that's a good question. There is a lot of literature that researched this question from the Detroit tire makers to recent materials science studies. The general answer seems to be that it depends. It especially depends on whether you mean natural rubber or things like "Pli-sac' or other synthetic sacs which last much longer. Temperature, moisture, oxygen, ozone levels, other chemical exposures, etc. all affect the life greatly. But the general recommendations which are very conservative, is that natural rubber, nitrile, neoprene & such have 5-10 year shelf life. Silicone has 20 year shelf life.