At some point we are hoping to be able to offer a compact and reasonable comprehensive booklet on plunger-filler repair. For the moment, here are some very basic instructions on dealing with thin-rod American-style plunger-fillers, such as Sheaffers and Wahl-Eversharps of the 1930s and '40s.
Replacement of the piston seal washer is straightforward, but note that correct diameter is critical. Too small and there will be no seal; too large, and the washer will catch on the barrel walls -- sometimes ending up torn right off the piston. In general, the perfect diameter is only slightly larger than that of the backing washer. Correct thickness is also essential for optimal functioning and durability. Piston washers can be cut from 1/32" thick rubber sheet, but original washers were thinner, and thinner sheet does work significantly better. We now sell sheet stock of the correct stiffness and thickness. A punch set can be bought ready-made, or can be fabricated from metal tubing available at any hobby shop. A little water as a lubricant helps insure a clean cut as the punch is twisted through the sheet. Centering the hole can be a bit tricky; a centering jig can be made quite easily out of a small section of metal or plastic rod, center-drilled to accommodate the center hole punch. The original Sheaffer washers came in three diameters: 9/32", 21/64", and 25/64", with a center hole diameter of 1/16".
The main obstacle for plunger-filler repair has been the packing unit, which may also be termed the shaft seal. Sticking a thick, stiff rubber plug into the barrel has been a popular stopgap, but has done little to turn around the plunger-filler's reputation as difficult to fix and prone to early failure. The best solution to date has been to drill out the original packing from the inside, using a 1/4" drill to open up the original closure washer and a flat-ended scraping tool (a sharpened screwdriver will serve) to clear away the rubber and felt bits inside. We have found that it is actually more effective to twist the drill by hand, since this allows the drill to be pressed forcefully against the closure washer which would otherwise spin and prevent the drill from catching. Turning the drill by hand also prevents overpenetration, allowing one to feel when the hard plastic closure washer is pierced and the drill working against the soft packing materials inside. The drill bit can be held in pliers, but a better alternative is to use a tap holder, or to clamp the bit in a bench vise and turn the barrel against it. A sharpened screwdriver is a useful tool for final cleanout of the packing recess. New packing material can then be pushed into place, again working from inside the pen barrel. One can either use alternating disks of rubber and grease-impregnated felt (the original solution) or a specially-sized rubber gasket. The packing chamber can then be closed off using 1/4" styrene washers, which you can either make yourself from hobby shop plastic stock, or order here. A tiny bit of solvent cement will give a quick and effective bond. In our experience, acetone evaporates too quickly to provide a reliable weld; MEK (methyl ethyl ketone) is much more satisfactory, though more toxic and taking longer to cure fully. Epoxy can also be used, but in any event, it is important to protect the inside of the barrel from marring -- a plastic or metal tube will help (a large-diameter drinking straw works well). Whichever packing method one uses, occasional application of silicone grease to the shaft will keep everything working smoothly.
A few additional notes:
Why cement the new closure washer in place? A screw-in closure would certainly be more elegant and would make repacking simpler. But given that the original packing units typically lasted decades, and that fluorocarbon rubber seals have an estimated life several times longer than the rubber seals used in the original packing units, it will probably be a long, long time before a newly-repaired packing unit will have to be opened up again -- long enough not to justify the additional cost and complexity of a screw-in closure.
Is a single solid gasket as good as the original packing of alternating rubber and felt washers? The main advantage of the more complex packing would seem to be its greater ability to retain lubricant. This would be of significant concern for a new design sold for general consumer use, but for a vintage pen being used by someone willing and able to wipe a bit of silicone grease on the shaft every now and then, the gasket is quite good enough.
A handy tool for installing the gasket may be made from a length of 1/4" diameter brass rod. Drill a 1/16" diameter hole in the end, into which you fit a piece of 1/16" hard steel rod a few inches long. The gasket can then be mounted on the steel rod, which will act as a guide to push it into place. If you cut a short length of brass tube of 1/4" inside diameter and slip it over the end of the brass rod, the tool can then be used to install the closure washer without risk of getting the solvent glue (acetone or similar) on the inside of the barrel.
With most plunger-fillers, the barrel of the pen is also the ink reservoir. Later Sheaffer plunger-fillers, however, are built differently, with a cylindrical ink reservoir inside of and separate from the barrel. These can be repaired without using a closure washer. The pen must be fully disassembled, including removal of the nib and feed unit from the front of the ink reservoir and removal of the plunger shaft. The packing unit can then be opened up from the back with a 7/32" drill and the packing chamber cleaned out with a pick. The rubber gasket is then pushed into place, again from the back. Because the hole is only 7/32" in diameter, once the shaft is in place, the gasket cannot be pushed out of place. This same technique can be applied to other plunger-fillers, working from the front (that is, from the inside of the barrel), but it is often quite difficult to clean out the packing chamber properly -- especially when the original rubber disks have hardened.
More plunger-filler info here.