(This page revised September 23, 2020)
With its “inverted” feed system, hooded nib, and reliable slip cap, the Parker “51” is arguably the most innovative fountain pen of all time. It is also among the most reliable. When it was introduced in 1941, the “51” was fitted with Parker’s proven Vacumatic filling system (shown in Anatomy of a Fountain Pen II: The Parker Vacumatic). In 1948, beginning with the Demi model, Parker refitted the pen with a new filling system that was simpler and more reliable although inferior in ink capacity. First dubbed “Foto-Fill,” the system soon found its name changed to “Aero-metric.” This article describes how the “51” works; the pen illustrated is the Aero-metric version that entered production in 1949 with the introduction of the all-stainless Flighter. The illustrations depict a pen with its proportions altered for illustrative purposes.
The first figure shows a view of the “51” with its cap in cutaway to show how the pen fits into the cap:
The “51” Slip Cap: Slip caps of the 19th and early 20th centuries could slip off inadvertently, exposing the pen’s nib to wick an ugly ink spot onto the user's garment. The “51” slip cap is precisely fitted and does not slip off. As shown in the next figure, the key feature is a clutch, a spring-steel set of “fingers” that press against a mating clutch ring on the pen body.
The pen slips into the clutch and is guided into position with its shell resting against the edge of the inner cap’s open end, sealing the nib area against evaporation. The springy clutch fingers press firmly against the clutch ring to hold the pen in the cap. The shell is sharply tapered so that no breather holes are necessary in the cap; as soon as the seal is broken when the pen is uncapped, air can enter the nib area.
The clip, with its attached tassie-like ring, is held in place by a threaded brass bushing that screws into the inner cap, and a celluloid “jewel” screws into the bushing to conceal it and provide an attractive finish. (Parker introduced this design on the Vacumatic in 1932, and variations of it have appeared on many Parker models since the “51”.)
The Aero-metric Filling System: The next figure shows the pen, without its cap, in cutaway.
The ink reservoir is an ordinary sac, but the sac is made of a transparent plastic material that Parker called “Pli-Glass.” Unlike the rubber diaphragm in the Vacumatic, the Pli-Glass sac is immune to the corrosive effects of ink and therefore does not ossify. Enclosing the sac is a tubular sac guard with an aperture cut in its side. A pressure bar spans the aperture, and the user removes the barrel and squeezes the pressure bar against the sac to fill the pen. Releasing the pressure bar allows the sac to draw ink in. The path of least resistance is the breather tube, through which the ink fountains into the sac. When the user makes the next squeeze, the path of least resistance is again the breather tube, which ejects its very small volume of ink back into the bottle during the squeeze and is ready to draw more ink upon release. Four to six squeezes are sufficient to fill the pen.
The breather tube in the first Aero-metric pens was sterling silver, and that is the design depicted here. Later, Parker switched to plastic.
There is a small hole at the extreme back end of the barrel. This hole permits air flow to equalize pressure inside and outside the barrel, preventing air expansion inside the barrel (and the consequent ejection of ink) due to warmth from the user’s hand or pocket.
The Ink Delivery System: The following illustration shows just the parts that make up the ink delivery system:
The actual feed in the “51” is the massive secondary reservoir, or collector. By holding a very large amount of ink immediately adjacent to the nib, the collector made it possible to use Parker’s super-fast-drying “51” ink. Any evaporation from the nib was replaced immediately from the collector; without this nearby supply of ink, the nib would have dried out in normal use. Ink is fed from the sac to the collector’s finned reservoir area (the tapered portion nearest the nib) through a capillary slit cut through the collector’s center spindle. The portion of the slit closest to the underside of the nib extends almost all the way to the front end of the collector, and a tiny spacer of hard rubber rod is inserted into a hole drilled along this slit at its back end to keep the slit from being squeezed closed as the collector is fitted into the threaded connector. Later, Parker changed the design to eliminate the hard rubber rod; instead, a small area of the collector was melted and deformed to wedge the slit open. For more than a decade after the pen’s introduction, “51” collectors were machined of clear acrylic, as shown here. In the early 1960s, Parker retooled the pen slightly, and a new injection-molded collector replaced the more costly machined part.
From the surface of the collector, ink travels along the outer surface of the cylindrical nib to the nib slit, and thence to the tip. The hard rubber feed provides a second capillary path on the inner surface of the nib, and it also supports the breather tube. The original “51”s feed has no ink fissure; but Parker later found that ink flow control was much improved by the addition of a single fissure. There is no air channel in the feed, only the fissure. Air passes into the sac through a broad air channel that runs along the periphery of the collector, in line with the top surface of the nib. (From the channel it passes around the collector body between fins and thence into the collector slit.)
The shell provides the enclosure necessary for the collector and nib; in a sense, the shell is also part of the ink delivery system because it forms the outer wall of the secondary reservoir. The shell is sealed to the threaded connector by an adhesive like shellac, so that the only path for ink and air to pass between the sac and the exterior is through the nib area. (In some versions of the “51”, there is a rubber O-ring at the joint where the shell and connector meet the clutch ring. Parker did not apply an additional adhesive to pens with this arrangement, but many modern repairers seal them as a precaution.)
For an exhaustive and highly technical exposition of the ink delivery system, refer to U.S. Patent No 2,612,867.
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 4, an ebook for your computer or mobile device.