Since I had a couple of Cross push-in converters laying around, I decided to see if I could modify one to fit the Geha. After some careful measurements, trial-fitting, and making it up as I went along, here's what I came up with:
The bottom of the converter (left in the photos) is made of a soft plastic. The last 3mm or so is narrower inside than the rest of that part, and forms the seal around the nipple inside the pen. The inside diameter was just under 2.9mm, whereas the nipple in the Geha is 3mm in diameter, so I enlarged it using a 3mm drill bit. I then used an X-acto knife to trim off part of the sealing area (as part of the overall shortening of the converter), leaving about 2mm of the seal. To ensure a good seal, I put a thin coating of silicone grease on the inside of the seal.
Next, I disassembled the converter and shortened the knob to 23mm by cutting off part of the top. As you can see in the photo, the piston screw can extend past the top if the piston is screwed all the way out. In actual use, the screw can't protrude more than a tiny bit, or the converter won't fit in the pen.
The photo below shows the modified converter next to an unmodified converter (which happens to be full of ink). The piston in the modified converter is almost at the point where the screw would start to protrude from the knob.
At this point, I could install the converter in the pen. All my measurements indicated that I should have been done, but I could not screw the barrel all the way down onto the section. I eventually determined that the metal sleeve that holds the knob in place was hitting a ridge inside the barrel. This ridge was there to keep the spare cartridge from moving around.
To resolve this problem, I did two things. First I disassembled the converter again and chucked the sleeve in a drill with the knurled section protruding. Using first a file and then some fine emery cloth, I slightly tapered the knurled part of the sleeve. Next, I used a very sharp 5/16" drill bit, held in my hand, to carve out the inside of the barrel. From my measurements, I should have used a 7.5mm or 19/64" bit, but I didn't have one and I determined that the outside diameter of the barrel was enough that I could safely use the larger bit. I went very slowly, turning the bit a few times and then test fitting the barrel on the section. I stopped as soon as I could get the barrel all the way on and back off again without pulling the converter out of the pen.
With the barrel removed, the converter is a bit wobbly because the outside diameter of the lower part is quite a bit narrower than the inside of the section. However, with the barrel in place, the converter is unable to move. By my calculations, this converter can hold about 0.5mL of ink when screwed out as far as will fit in the pen, which compares favorably with a Geha, Cross, or short international cartridge, and is about half of what a Sheaffer or long international cartridge holds.
Now I just need to make a few cosmetic improvements to the pen, such as removing the teethmarks from the cap (no idea how I'm going to do that), and finding something to put into the top of the cap (there's something missing, but I don't remember what used to be there, so if you know, please let me know). There's also some damage to the nib, including some pitting on top next to the slit, and a nick in one side, but neither of these blemishes seem to affect its writing ability. The pen should be good for another 38 years!
Edited by stefanv, 16 February 2011 - 03:02.