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How to Repair a Broken Sac Nipple or Parker “51” Aero-Metric Coupler

(This page revised August 7, 2020)

Reference Info Index | Glossopedia  ]

Fountain pen


Occasionally, you will be faced with a sac-filling pen or a syringe-filling Morrison Patriot whose section has lost all or part of the sac nipple. With the Patriot, the filler is sometimes so reluctant to come off the nipple that it takes the nipple with it when you separate it from the section. This failure is more likely to occur on transitional Patriots that have hard rubber sections.

The problem crops up in a different way with the Parker “51”: When Parker switched from machined acrylic connectors to molded plastic ones for the Aero-metric “51”, it soon became apparent that the Pli-Glass sac was made using a formulation that outgassed plasticizer. It softened the plastic of the connector’s nipple irreversibly. The solution for Parker was to change the formulation of the sac material, but there are still thousands of "51"s around with molded connectors and the earlier sacs.

If you eliminate junking the pen as a viable way to handle the situation, you’re left with replacing or repairing the section or connector. Replacing the section is easy if the pen is a common type, such as an Eversharp Skyline or a Waterman 52; but for more esoteric pens, such as the Parker Safety-Sealed button filler illustrated at the top of this page, there is no meaningful supply of parts. With the “51”, the repair is so easy that replacing the connector seems like overkill and a waste of a good part. This article shows how to salvage the existing section or connector by replacing the broken or missing nipple.

Tools Required

Supplies Required


At first glance, the “junker” pen shown above looks like an eyedropper-filled Parker Lucky Curve. When opened, however, it turns out to have had a sac:

Pen opened

A closer look at the back end of the section reveals that it had once had a sac nipple. At the back of the barrel, there is a well-fitting blind cap. This pen is a Safety-Sealed button filler made sometime between about 1912, when Parker started making button fillers, and about 1916, when Parker engineers combined the button filler with the screw-cap feature of the Jack-Knife Safety pen, and it is very much worth restoring. Here is the section by itself:

Pen section, broken

Taking Measurements

Absent a well-stocked source of replacement sections, restoring the pen to service requires making a new sac nipple for the section. Before you can do that, you need to know how big to make the nipple. Two critical measurements are required.

Preparing the Section

The sac nipple on the pen illustrated here had been cleaned up after it broke off, but in most cases not all of the nipple will break off neatly, and the broken edge will be rough. The first step in the repair is to clean up the end of the section. This task can be accomplished by other methods, but the quickest, easiest, and most accurate way to do it is to use a lathe.

If the pen has a screw cap, the section will probably have a table (the flared-out flat end adjacent to the nib, where the inner cap seals when the pen is capped), and you can chuck it into the lathe quite easily. Wrap two layers of blue painter’s tape around the section to protect its surface from the chuck jaws, applying just short of two full wraps. Fit it into the chuck, positioning the place where the tape layers overlap so that it is between jaws; this ensures that each jaw will clamp on two layers and thereby keep the section centered. Carefully tighten the chuck jaws, sliding the section in or out as needed until the chuck clamps both the table and the base of the section as shown in the following drawing. Note that for illustration the section in the drawing has a table:

Section in chuck

(If the section does not have a table, as the one shown in this article does not, then wrap a narrow strip of the blue tape around the nib end of the section multiple times to build it up to very slightly more than the diameter of the base of the section, and then apply the two-layer wrap. When you clamp the section in the chuck, the tape will compress slightly to approximate the diameter of a table.)

With the lathe running at about 360 RPM, adjust the cross-slide so that the chisel’s tip is just within the diameter of the section’s bore. Now operate the main carriage to bring the facing chisel slowly toward the section as shown here:

Cutting away the broken nipple

Slowly cut away the rough bits of the nipple until you reach the flat surface at the end of the portion that fits into the barrel. Leave a finished surface there.

With the back end of the section finished, the next step is to drill a hole into the same end that is as close as possible to the desired outer diameter for the nipple. If there is not an exact match, go smaller instead of larger. For our example pen’s diameter of 0.325" (8.27 mm), the closest drill size is size P (0.323" or 8.20 mm). Mount the drill in the lathe’s tailstock, and bore about 1/4" (6.4 mm) into the end of the section.

Boring the section

Here is the back end of the section after it has been bored for the nipple:

Pen section, bored

Making the Nipple

For projects of this type, you can use the inexpensive small-diameter “hard rubber friction rod” that is available from educational scientific supply houses. One source for these rods is HomeScienceTools.

Remove the section from the lathe and chuck the hard rubber rod up in its place, with only about 1/4" (6.4 mm) exposed.

The next step is to drill the bore of the new nipple. It’s easy to just slap a drill into the tailstock and drill away, but an ordinary drill is not rigid enough to ensure that it will not wander and drill off center when brought up against a flat surface, especially one that is not perfectly true. To keep the drill from wandering, drill a starter hole using a center drill, which is much more rigid and much shorter than an ordinary drill and will not wander. Mount the center drill into the lathe's tailstock and drill a starter hole. The center drill that I specify should provide ample clearance for the point of the drill you will use to make the bore of the nipple.

Drilling a starter hole

Mount the drill that you selected for the nipple’s bore into the lathe's tailstock and bore about " (19 mm) into the end of the rod:

Boring the new nipple

Re-chuck the rod in the lathe, with about " (19 mm) exposed. Turn the drill around and remount it as far into the tailstock as it will go, leaving its plain end exposed, and run it into the hole you just drilled until it bottoms. Back it out one turn of the tailstock’s crank (about 0.050" or 1.25 mm), and lock it down. The drill serves as a stabilizer to keep the hard rubber rod from wobbling as you turn its outside surface.

Turn the outside diameter of the hard rubber rod down to a diameter almost the same as the diameter you measured for the outside of the nipple; just a little larger is good, but do not go smaller than the desired diameter. Back the tailstock off and test to see whether the rod fits into the section. If it fits snugly but without force, life is good. If it doesn’t, set the mandrel again, advance the cross-slide 0.0005" (0.013 mm) to turn the rod’s diameter down by 0.001" (0.026 mm) overall. Repeat the checking and turning process until the section slips onto the rod snugly but without force.

If you turn the diameter down too far, the nipple will be a sloppy fit in the section and might end up off center, or come loose due to an inadequate adhesive bond. If you do not turn it down far enough, you risk splitting the section.
Attempting to turn the rod down in one sweep is unwise; it’s very easy to go too far.
Turning the nipple

Using the cutoff tool, cut the nipple about 1/2" (13 mm) in length. The mandrel will keep it from flying away and getting lost when you have cut through:

Cutting off the new nipple

Use the sandpaper to break the exposed sharp edges at the ends of the nipple. Here are the finished nipple and the section:

New section nipple

Apply shellac to the inside of the bore you made in the section and on half the length of the nipple, and insert the shellacked end of the nipple into the back of the section. Clean off excess shellac on the outside, not worrying about the inside, and leave overnight so that the shellac can dry.

On the next day, choose the largest drill that will go through the section’s original bore and run it by hand through the full length of the section and nipple. If the drill stops partway through, rotate it to cut away the excess shellac that is in its way. Pass it back and forth several times to ensure that the bore is clean so that the feed will be able to pass through without binding. (If the feed binds in the new nipple, installing the nib and feed can break the shellac bond and drive the nipple out of the section.)

Assemble the section, feed, and nib:

Pen section, assembled

Install a new sac, let the shellac dry, apply talcum powder, and assemble the pen.

Using This Technique to Repair a Parker “51”

When Parker started making the Aero-metric “51”, the material that was specified for the Pli-Glass sac turned out to be the wrong material. It was fine for use with the acrylic threaded connector parts Parker first used, but when production changed to injection-molded connectors, things got ugly. Over time, the sac would outgas a plasticizer that softened the molded parts. When the company’s engineers discovered this problem, they changed the sac formulation. To salvage a “51” with a softened nipple, cut off the nipple, chuck the remaining part of the plastic connector piece in the lathe, and true up the end as described above for an ordinary section. Bore " diameter about " deep into the back of the connector. Fuse in a " (13 mm) length of " polystyrene tubing using a liquid plastic welding solvent (e.g., “Same Stuff” from MicroMark) applied both to the bore you made in the connector and to half the length of the tubing. Allow the solvent to flash off for an hour, then use sandpaper to break the sharp edge of the cut end of the tubing, and install a new sac.

Connector with new nipple
The preceding paragraph does not give metric equivalents for the bore and tubing diameters because I have no information on styrene tubing made to metric dimensions and because the diameters are critical if the fusing process is to work and a standard Pli-Glass sac is to fit.

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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