I've used the Cross Ion for three years and love the shape, function and lanyard. (I don't like the sharpish edges where it separates in the middle, however.)
But what I really hate is the lousy, leaky, short-lived, slug trailish gel ink cartridge!
For the longest time I've wanted to stick a Fisher cartridge in it somehow, but didn't know if Fisher made a cartridge small enough, or whether a retrofit would be possible. The other day I was in a Staples office supply and saw a Fisher "Universal" pressurized cartridge "for multi-action pens" and noticed that it was shorter by a hair than the Cross Ion cartridge. It occured to me that with a little ingenuity this cartridge could be installed in the Cross Ion.
And guess what, it could and I did. It wasn't too hard, and here's how.
Remove the Cross cartridge from the Ion. Take off the white plastic tail cap from the cartridge and set it aside. Over the kitchen sink, yank out the metal tube (containing the ball point) from the cartridge body using a pair of pliers, applying a twisting motion.
Still over the sink, blow out the ink using your lips over the open tail end of the cartridge. The bulk of the ink will blow out easily. However there's some clear goop that plugs the hole once all the ink is gone; blowing out this stuff takes some persistence (it took me a couple minutes of blowing--you might want to experiment with an air compressor or bike pump, etc.)
Once the ink and the clear goop are gone, rinse out the cartridge with hot running water. You might also want to twirl a Q-tip around inside to clean better.
Now put a 3/32" drill bit on your drill or rotary tool and bore out the hole where the old metal tube and ball used to be.
You are now ready to insert the Fisher cartridge. Remember this is the "universal" type cartridge, which has sections that snap off so the cartridge can fit a range of other pens (it's apparently designed for multi-action pens). Snap off both extra sections.
Now insert the new Fisher cartridge through the tail end of the old plastic cartridge. Push the new cartridge firmly into the plastic shell until the end of the Fisher is about 1/16" below the tail end of the tube. Now the business end of the Fisher is right where the old Cross ball-and-tube used to be.
Now take the white plastic tail cap that you set aside. Wad up a little piece of paper kitchen towel and stuff it into the recess on the inside of the tail cap. It should just fill the little cavity. (Or stick a tiny rubber washer in there, or maybe even a very small set screw.) Put the tail cap back on the cartridge, ensuring that the metal end of the Fisher cartridge will find a nice center.
Pick up the bottom half of the Ion pen that you set aside when you pulled out the cartridge. What you want to do now is expand the hole that the ball point sticks out of, by just a hair. You need to do this because the diameter of the Fisher tube is just slightly greater than the old Cross tube. (The tip of the Ion appears to be made of chrome plated brass, but anyway . . . .) Using your rotary tool and an appropriate grinding bit (I used a small conical diamond bit about 2.5mm), expand the hole. There's no precise measure here, just grind smooth and regular until the Fisher tube stops catching in the hole.
The last step is the trickiest. You will need to file down the plastic at the narrowest section of the Cross cartridge, since this section expands a little following the modification and would otherwise impinge on the spring. You need to file this section down using a small hand file until the diameter is .134" or less. (If you don't have measuring calipers just take it down until the plastic stops impinging on the spring.) This plastic doesn't file down easily because it's so slick, so you might want to use a diamond file.
Finally, put the new "Cross-Fisher" cartridge back into the Ion. You are done!
This is now the ultimate pen! At least for me. (I admit I've never used the telescoping Fisher Bullet pen but it looks too skinny to me.)
As a sub-note, I want to mention that I did the same mod using Staedtler Multi-action Pen Refills, and also Sterling brand (as found at Target), and it worked just the same.
Good luck. Any questions?