It is commonly said that the Parker 51 Aerometric is virtually foolproof, and that most of them found "in the wild" can be put back into use after nothing more than a good flushing with cool water. Yet many unrestored Aerometric-filling 51s need more: at minimum, disassembly and a thorough cleaning, and often, replacement of the breather tube.
While many old Aerometric 51s will suck up water (or ink) when their sacs are squeezed, that does not mean they will fill completely or write properly. The fine-finned collector is superbly adapted to wicking up excess ink by capillary action, but as a result can be very difficult to get completely free of old, hardened ink encrustations. The sterling silver breather tube is often clogged or badly corroded (the examples above are typical), allowing the sac to fill to only a fraction of its capacity and shedding bits that do nothing to improve the reliability of ink flow.
Removal of the remains of an old breather tube can be a chore. The best method we have found is to thread a miniature tap (.90 UNM or 000-120) into the stub of the tube left in the feed, then apply heat until the stub can be wiggled out. The hard rubber of the feed may end up slightly distorted from the heat and pressure, but will return to its original dimensions upon reapplication of heat without pressure.
Original replacement breather tubes are very hard to come by. Some repairmen have been using stainless steel tubing originally made for hypodermic needles. A simpler alternative is Teflon, which is supremely resistant to chemical action and easy to work with. We now sell Teflon tubing in two sizes, one for Aerometric 51 breather tubes, the other that can be used for Vac-filling 51s as well as Vacumatics, Skylines, and other pens). Both sizes can be cut with a sharp knife, but we recommend inserting a length of brass or copper wire (AWG 20 = .032") before cutting the thinner tubing to prevent flattening.
Note that original Aerometric breather tubes have a small hole drilled through one side about 6 mm down from where the tube enters the feed. The purpose of this hole is to equalize internal and external air pressure, preventing the expulsion of ink during flight. Drilling this hole in a metal tube requires a special jig, but Teflon tubing can easily be pierced with a small hand-held twist drill. The overall length of the breather tube should be approximately 7 cm. Because of the very different surface properties of the two materials, it is probable that a metal tube will outperform a Teflon tube when it comes to pressure equalization. In most cases, however, the difference will not be perceptible, and in fact later 51s abandoned the vented metal breather tube in favor of plain plastic.
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