Thought it was worth giving this useful information a bump. My own efforts:
Pilot Petit1 on the left, the other is a Junior K. The Pilot takes a special short cartridge. I actually made the converter a little longer than I'd estimated, and looking through the transparent barrel, it is exactly the maximum length it can be. Works great. The Junior K takes international short cartridges. Slight problem there, although it works. The inside diameter of the barrel is so small that the added width of the sac on the the outside of the cartridge makes it a tight fit. Over time that might wear the sac down or make it come loose. It's good for now, though, and I probably won't use the Junior K much after it's emptied anyway.
It's almost a shame that I don't have more pens where I need to do this.
Pilot Petit1 on the left, the other is a Junior K. The Pilot takes a special short cartridge. I actually made the converter a little longer than I'd estimated, and looking through the transparent barrel, it is exactly the maximum length it can be. Works great. The Junior K takes international short cartridges. Slight problem there, although it works. The inside diameter of the barrel is so small that the added width of the sac on the the outside of the cartridge makes it a tight fit. Over time that might wear the sac down or make it come loose. It's good for now, though, and I probably won't use the Junior K much after it's emptied anyway.
It's almost a shame that I don't have more pens where I need to do this.