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How to Restore an Onoto Plunger Filler

(This page revised June 5, 2017)

Reference Info Index | Glossopedia  ]

Fountain pen
Fountain pen

Virtually all collectors and repairers of vintage pens are familiar with Sheaffer’s great plunger-filling pens of the 1930s and ’40s. But the average U.S.-centric collector may not know that Sheaffer didn’t invent plunger fillers. The original plunger filler came from England. Invented by a mechanical engineer, tinkerer, and sometime vaudeville performer named George Sweetser, it came rolling out of Thomas De La Rue & Company’s factory in 1906, and it looked like the upper photo above. The lower photo shows a pen from the decade before World War II. Except for the change in feed design, the two pens are essentially identical internally.

Among plunger fillers, the Onoto is probably the easiest to repair; but there are some sneaky pitfalls waiting to trap the unwary repairer. Many who attempt this repair take the “easy” way out, fabricating a new cork shaft packing and then splitting it so that they can assemble it without removing the blind cap. That technique is actually more difficult than doing things right, and it also produces an inferior result because it relies on squeezing the new packing to keep it from leaking through the split and because the packing, being a cork, can dry out as the original did. This article explains how to do the job right, by removing the blind cap and using an O-ring for a replacement packing.

Tools Required

Parts Required (see footnotes for more information)

Supplies Required (see footnotes for more information)

It is possible to disassemble one of these pens without section pliers, but it is difficult, and the risk of damage is high. I recommend the section pliers shown here:

Section pliers

You will need a pin punch for driving out two small hard rubber pins in the pen. The photo below shows my punch. Make the tool from a 2" (51 mm) length of " (13 mm) hardwood dowel and a couple of pieces of stainless steel music wire. One piece, a 1" (38 mm) length of 0.039" (1.0 mm) wire with its end nicely flat and perpendicular to the wire’s axis, is the punch. Drive that piece of wire into a same-sized hole drilled into the end of the dowel, leaving about " (13 mm) exposed. (If the hole you drilled isn’t deep enough, you can cut off the excess and true up the end after you finish the rest of the procedure.) If at all possible, do the drilling on a lathe; it’s almost impossible to hold things straight enough by hand. The other piece is 0.032" (0.8 mm) wire, and it goes through a same-size hole drilled laterally through the dowel to serve as a backstop for the punch. Drill this hole with extreme care to ensure that you line it up with the punch wire. Insert the transverse wire and then form one of the wire’s exposed ends into a square U shape and pound it the rest of the way into the dowel to keep it in place. Grind the last exposed end off flush with a rotary tool.


I use a jeweler’s forming block as a steady rest when I’m knocking pins out of pens. Forming blocks come in different sizes and shapes. The one I use is flat, about 1" (25 mm) thick and about 4" (102 mm) by 5" (127 mm), with half-depth round grooves of various sizes cut across it on both sides.

Forming block

Disassembly, Cleaning, and Seal Removal

Before you attack your first Onoto, you should know how it’s built inside. Here is a cross-sectional drawing of the pen to show you the layout of the components.

Fountain pen diagram

The picture below shows how to knock out the pin that passes through the blind cap to secure the plunger shaft. The first thing you have to do is to find the exposed ends of the pin. It’s made of hard rubber, just like the blind cap, and the exterior surface of the blind cap was finished after the pin was driven through — this can make it difficult to find the pin. Look hard enough, however, and you’ll eventually find it. You can see from the photo here and from the cutaway drawing above that the pin goes through the blind cap very near the back end of the blind cap; knowing this will help you to find the pin.

Knocking out a pin

Once you’ve found the pin, lay the pen barrel into the most closely fitting larger-size groove in the forming block or, if you don’t have a forming block, on a flat surface such as the top of a wood block cut from a 1×4. Use blue painter’s tape to tape the barrel down securely with the blind cap hanging a little over the edge. So that you can see the shapes of things, this photo shows the pen projecting farther past the edge of the block than it should; ideally, the pen body should be entirely in the groove, with just the blind cap hanging over. As you’re taping the pen down, orient it with the pin through the blind cap aligned vertically so that you can drive it out. Line up your pin punch with the end of the pin, and tap with the jeweler’s hammer. Check to see if the pin has moved, and repeat the careful taps until it does move. Then you can drive it out.

With the pin out, unscrew the blind cap from the barrel and then unscrew it from the plunger shaft. Note that the blind cap screws onto the shaft with a left-hand thread. Turn the blind cap clockwise to unscrew it! You might need to use a little heat to get the blind cap to move, but once it is loose, it will screw off easily.

Removing the blind cap

Unscrew the section from the barrel. This joint is right-hand threaded; unscrew it in the usual way, by turning counterclockwise.

Removing the section

Now you should be able to push the plunger shaft out of the barrel from the rear. The photo below shows the plunger after it has been removed; the bulbous end was toward the nib.

The plunger, removed

The cone-shaped part near the end of the shaft is labeled Plunger Head in the cutaway drawing. As the drawing shows, it can be removed from the shaft — but there is no need to take it off, and it’s always better not to take something apart unless you must do so. Instead, remove only the long skinny thing at the very front of the shaft (the green part labeled Shaft Nut in the cutaway drawing above). Like the blind cap at the other end of the shaft, this part is secured by a small hard rubber pin. Locate the pin and drive it out with the pin punch. This task, as with knocking out the blind-cap pin, is most easily accomplished if you have a forming block to rest the shaft in.

Driving the plunger pin out

Remove the shaft nut from the shaft (below). Some pens have a plain unthreaded shaft, while others have a threaded shaft (left-hand thread).

Removing the shaft nut

Now it is time to remove the shaft packing at the back end of the barrel. First, you must remove the threaded hard rubber ring that secures the packing in place. There is no slot or other ready means provided to unscrew the ring. The best way I have found to do this is to use bench knife as if it were a screwdriver, as shown in the following photograph. The knife being used here is a jeweler’s bench knife with a sheep’s-foot blade; most ordinary pocket knives will work as well, so long as the blade you use is broad enough that it catches on the edges of the ring instead of slipping through the ring. To unscrew the ring, press down just enough to keep the knife engaged and not slipping inside the ring. The ring has a right-hand thread. The photo below shows the ring removal operation.

Installing the retaining ring

Now use an appropriate dental pick to dig out the cork (below). Do this carefully to avoid scoring or scratching the sides of the recess in the end of the barrel; a scratch there can compromise the ability of the replacement packing to make a tight seal. Clean out any bits of debris; this area should be scrupulously clean.

Picking out the cork

Clean the pen thoroughly. Use the ear syringe to force clear cool water through the barrel and through the section. Then flush the section with pen flush or an ammonia solution.

Restoring the Seals

Start with the plunger gasket. Look for a circular groove in the end of the plunger head (below). Early models like the one being repaired in this article will have this groove, but some later Onoto pens have a plunger head with a flat end and a washer here instead. The washer has an inside diameter large enough to leave the required space.

Plunger head showing groove

The special cup-shaped plunger gaskets come in sizes for standard pens and for the Magna. Install a new plunger gasket onto the end of the shaft, making sure that the boss at the gasket’s smaller end fits into the plunger-head groove (or between the washer and the shaft), and then reinstall the shaft nut. Line up the holes in the nut and the shaft, and reinstall the small hard rubber pin. Be careful here; if you do not have the holes lined up perfectly, you can break the pin, the shaft, or the shaft nut. When you have finished this assembly, it should look like the photo below.

Plunger head reassembled with new gasket

Use a Viton® O-ring, size M1.5×3 or M1.5×3.5 as appropriate, as a replacement for the cork you removed. The O-ring will give better performance and longer life (and will not shrink and dry out if the pen is left empty). Apply silicone grease fairly liberally around the inside of the recess in the end of the barrel, avoiding the threaded area near the opening, where the hard rubber ring will go back in. The photo below shows this step; I’m using a length of ” brass rod with its shaped to a hemisphere and polished smooth.

Greasing the barrel recess

Insert the O-ring.

Inserting the O-ring

Use the appropriate size of styrene tubing as a tool to seat the O-ring all the way into the recess. It will be a tight fit, and you will need to be careful that it seats flat instead of remaining cocked in the recess.

Seating the O-ring

Cut a ” (4.8 mm) length of the ” (4.8 mm) or " (5.5 mm) rigid styrene tubing. Deburr its ends.

Cutting the spacer

Insert the piece of tubing into the recess on top of the O-ring (photo below). The O-ring does not occupy as much space as the original cork, and this tubing is a spacer to take up the rest of the space in the recess so that the O-ring will not slip back and forth, risking a failure of the seal. Do not get clever and use two or three O-rings instead of one O-ring and the plastic tubing; with more than one O-ring, the plunger cannot slide freely enough.

The spacer in position

Now reinstall the hard rubber retaining ring using the bench knife as you did when removing the ring. Tighten the ring down just enough to keep it secure; overtightening it will squeeze the O-ring and make installation of the shaft more difficult — and will also make the filling action stiffer. If the action is too stiff, the user can bend or even break the shaft.

Apply silicone grease to the end of the shaft adjacent to, but not on, the threads. See the photo below.

Greasing the shaft

Now slip the shaft, threaded end first, into the pen from the front. Find the opening in the O-ring by feel, and then screw the shaft (left-hand thread) into and through the O-ring.

Reinstalling the shaft

Apply a rosin-based non-hardening thread sealant on the section’s threads, and screw the section tightly back into the barrel (photos below).

Reinstalling the section

Clean off any sealant that comes oozing out of the joint as you tighten it. You can use heat, very sparingly, to soften the sealant and then clean it off with a cotton flannel rag.

Testing the Pen

Test the pen’s filling action. The plunger should pull upward very easily and be more than a little stiff going down. Try it with water; even a short ladies’ pen should take up 1 ml or more of water, and bigger pens should take up more. If it doesn’t fill, you may need to use a slightly larger cup gasket. On the other hand, if the plunger comes up very stiffly, you should probably go down one size on the cup gasket. (Note, however, that the plunger will come up more easily when the barrel is wet inside than when it is dry.)

Finishing Touches

The final step is to reinstall the blind cap (left-hand thread). Screw the blind cap onto the shaft until the holes in the blind cap and the shaft line up perfectly, and and stake the blind cap in place by reinserting the hard rubber pin. The pin is very delicate! Be sure you have the holes properly aligned; you can see light through them when they’re half a turn out of line, and if this is the case when you drive the pin in, you will break the pin. You can use your pin punch to verify the alignment; if the punch will not go through the hole, figure out why and correct the problem.

When you install the pin, try to put it back in an orientation that matches the original as closely as possible. Frequently, Onoto will have imprinted a model number on the blind cap in such a way that the imprint overlaps one end of the pin. You can use the fragment of imprint that is on the end of the pin as an indication of how the pin should be positioned. The photo below shows the pin being inserted.

Installing the blind cap pin

Reseat the pin into its original position. Tap gently! Ten gentle taps are much better than one hard tap that breaks the pin.

Seating the blind cap pin
As noted, the hard rubber pin securing the blind cap is delicate. If you break it, you can fashion a replacement from 1.2 mm carbon fiber rod. The hole in the blind cap will probably be just too small to accept the rod without your reducing the rod’s diameter a little by working it on the rotary tool with a fresh cutoff wheel. Do this carefully so that you do not create a tapered or keg-shaped rod. Test frequently to ensure that your rod is a snug fit but that it will go. Make the rod a little too long. Once you have a good rod, install it and then carefully grind away the excess part of the rod so that it is flush with the blind-cap surface at both ends.

With the pin in position, heat the areas of the blind cap near the ends of the pin a little (not necessary if you replaced the pin with carbon fiber). This will soften the pin and allow its ends to resume the dent-free shape they had before you drove the pin out. The final step is to sand the ends of the pin very lightly to blend them in with the surface of the blind cap, and polish the sanded area with a little Simichrome on a cotton flannel rag. Be sure to clean off the Simichrome completely when you’ve finished polishing.

  1. The two O-ring sizes listed are to fit standard or Magna pens. I recommend Viton because of its excellent resistance to chemicals and to wear.

  2. Nominally, there are two sizes of cup gaskets, one for standard pens and one for the Magna. In actual practice, however, there is enough size variation among pens that Jim Marshall, who supplies these parts, offers several parts that vary very slightly in size. I recommend purchasing a set of all the sizes for the pen you will work on; the gaskets are not inexpensive, but it’s more expensive — not to mention frustrating — to put a pen together only to find that the gasket isn’t quite large enough and that the pen will consequently not fill.

  3. Styrene tubing is used by model railroaders and architects to build scale models. It is available on the Web or at hobby shops that cater to model railroaders.

The information in this article is as accurate as possible, but you should not take it as absolutely authoritative or complete. If you have additions or corrections to this page, please consider sharing them with us to improve the accuracy of our information.

This article is also available as a chapter in The RichardsPens Guide to Fountain Pens, Volume 2, in either of two printed versions or as an ebook for your computer or mobile device.

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