That's actually one of the things that makes it so liked. The F-701 takes a Fisher refill quite nicely, but you need to prep it just a bit. There are three common ways to deal with this. Unscrew the writing tip of the 701. Use a paper clip to push out the plastic piece. The first way is to stretch it. I've had good luck with some pieces working them over the Fisher refill and then working them backwards over the narrow end of the thicker Zebra cartridge that came with the pen. If you're lucky, doing this and maybe letting it sit for 10 minutes on the Zebra cartridge permanently stretches it enough to where it will work smoothly with the Fisher refill. This is my best-case preference. Then there is the fold-over method where you fold the narrow tip of the plastic piece inward, again working it backward over the Zebra cartridge to stretch it. This worked for me on pens where the plastic piece simply refused to stay stretched after the spring pushed against it in the tip. This is my second choice method. Then there is the method whereby the modder uses a razor or other implement to cut off the narrow tip of the plastic piece. Its simple and it works first time, but I don't prefer it because, in my experience, its leaves the pen nib just a little bit sloppy. Not much, but enough to where you might notice a click when you set the tip to paper to write. The other two methods seem to leave the tip suitably reinforced to where there is no play. I've made six of the F-701 SS Space Pen mods so as to have one available wherever I might need it, ie. one in my coat, one in my car, my desk, my portfolio, etc. Just today I used the one in my portfolio over the course of several hours. Seriously, if you're into pens and even mildly interested in performing a simple mod, the F-701 SS Space Pen mod is a very rewarding, fun little project. It basically makes a decent $7.00 pen into a sharp looking, reasonably bullet-proof, write anywhere Space Pen for around $15-$16.