Anyone who has tried their hand at repairing Sheaffer plunger fillers figures out pretty quickly that getting the 0-ring and the retaining washer down in the barrel is a pain in the tush, especially if you have any kind of adhesive on the washer. How in the world do you get that thing straight down the barrel and into the hole you've drilled? About 12 years and thousands of pens ago, I ran in to the same problem and came up with a simple tool. After reading comments from people here on FPN, I concluded that the average amateur needed the tool, but I had no desire to make them to sell. So at the 2017 Chicago show I suggested that Dale Beebee to come over to my table and bring his calipers with him. A couple of months ago Dale released the tool that is a copy of my original that he as labeled the "RZ." Let me be clear that I have received, and will receive, nothing from Dale for the tool. Consider it a gift to the pen community.
The tool is simple - a piece of stainless rod with the ends machined, one to hold the 0-ring, one to hold the retaining washer. The pins are big enough to hold the pieces while inserting them, but loose enough that the 0-ring or washer are left in the barrel when the tool is removed. Put the 0-ring on the end, guide it down the barrel and into the hole you've drilled to remove the old packing unit and press it home. The same for the retaining washer, but you put your solvent or adhesive on the edge of the washer first, then press it snug in the hole on top of the 0-ring, and you're done. Its made out of stainless so that you can bump the end with a rawhide hammer to drive the retaining washer home if you need too.
For more information, visit Dale's website Pentooling.com
This is the tool:
The narrow end holds the 0-ring
The wider end the retaining washer.