Japanese Jumbo repair notes

Japanese Jumbo pens were produced from the 1930s up through the 1950s. Although some were made to a very high standard with carefully applied urushi, gold filled trim, and solid 14K gold nibs, most were cheap novelties made for export (none was ever intended for use by the arthritic -- an odd and relatively recent American collector myth). Even these pens, however, were solidly made using massive chunks of hard rubber finished with real urushi lacquer. Their main weakness is their metal trim, which was given the thinnest of gold wash coatings, guaranteed to wear off with the slightest handling.

Most Japanese Jumbo pens are dropper-fillers ("eyedroppers"). Don't be fooled by the center shaft attached to the end knob; that is an ink shutoff valve design that is characteristically Japanese, without any piston seal at the end of the shaft to allow the assembly to work as a filling system. In most cases the original cork packing seal is no longer sound, and if the pen is filled it will leak around the shaft. To replace the seal, you will have to start by unscrewing the knob from the shaft. This is typically the most difficult step of the repair procedure.

The threads are left-handed and the joint is glued with a form of mastic. With a pointed tool, carefully chip the mastic away from where the shaft enters the knob. This will allow you to apply isopropyl alcohol to the joint so that it can soak in. Repeated applications may be necessary, or you can put a small bit of cotton around the joint and saturate it with the alcohol. Screwing the knob part way onto the barrel will keep the alcohol from evaporating too quickly. Denatured alcohol can be used, but will evaporate more quickly and is more toxic (don't let it get on your skin).

Heating the knob from the outside with a heat gun may be necessary. Once the area of the joint is only slightly pliable, gently twisting the knob very slightly back and forth can break the bond of the threads just enough to allow the alcohol to enter. You do not want to twist too far or too hard with the hard rubber heat-softened, as the shaft is easily broken in that state.

Once you have successfully unscrewed the shaft, you will want to clean out the barrel interior thoroughly. Then plug the shaft hole, stand the barrel upright, and fill it with a bit of water. What you are doing is soaking the threaded joint providing access to the packing unit.

Empty and dry the barrel, removing the plug. Take a triangular scraper and push it firmly into the shaft hole and turn it counterclockwise, unscrewing the threaded closure washer. Using the right amount of pressure may take some practice -- you want to use just enough to keep the scraper from slipping (more on the use of scrapers for this purpose here).

Since the basic design of the ink-shutoff dropper-filler as adopted in Japan was taken from the Onoto plunger-filler, it's not a surprise that the dimensions of both shaft and packing compartment follow the Onoto standard as well. Premade Onoto cork seals can be used in Jumbo packing units, or you can cut your own, or use O-rings. Clean out the packing compartment, put in a new seal, and reverse the above directions to reassemble. 

Paradoxically, the less common lever-filling Jumbos are not as straightforward to get working as the ink-shutoff dropper-fillers. I'm afraid I don't have a photo, but these pens originally came with a truly outlandishly shaped rubber sac big enough to fill up the cavernous barrel and also necked down enough to fit the tiny section nipple.

The pressure bar is one-piece -- a true J-bar by the term's original definition. The thing is, the lever's throw isn't enough to push the pressure bar all the way down to the far side of the barrel interior. It's not even close, even with the thick walls of the original sac. When new, these pens would not have been able to empty their sacs fully. And resultingly, would not have been able to fill their sacs fully, either. Not optimal, in that fountain pens are inherently prone to irregular ink flow when they have more air than ink in their reservoirs.

In order to get this pen working, I built up the diameter of the sac nipple with a short cutting from a trimmed sac, then attached the largest sac I could find, a #24 necked. I then glued in a spacer opposite the lever, allowing the pressure bar to flatten the sac rather than just push it aside.

Note that many if not most J-series Esterbrooks have a similar internal spacer, though the Esterbrook pressure bar is a far superior two-piece design and not a one-piece J-bar. This now allows the Jumbo to fill -- though it is still best considered a novelty item and conversation piece rather than a practical everyday writing instrument.