The Zebra F-701 pen is a bit of a surprise for its price range of about $7. Stainless steel barrel, knurled grip... it just looks cool for an off the shelf pen.
What many people have found out is that you are able to swap in a Fisher Space Pen refill to the F-701 pretty easily, but there is another mod to swap out some of the hardware of the Zebra F-402.
Why do this? Well, the only downer to the F-701 (besides the Zebra ballpoint refill) is that the click mechanism is made of plastic. An all metal-ish pen and they choose to put in plastic instead. What is odd is that the F-402, a less expensive pen, has a brass mechanism. This is where the hack comes in...
First things first, we need to remove the plastic click mechanism from the F-701. I've found this easiest to do with a pair of needle nose pliers as I discard the part. I know, I know, I am throwing away perfectly good pens to do this hack...
Second, and a bit more tricky, use a lighter to remove the brass piece from the F-402. I had a blow torch laying around my shop while shooting this, but a lighter is better. There is a pressure fit plastic sleeve that threads the brass mechanism into the barrel; our goal with the lighter is to heat up slowly the end of the pen to melt the plastic. If you have a pair of non marring pliers, use them instead. The reason we don't want to go clamping down on the clicker is because we don't want to scratch it. (makes the pen a bit unsightly)
It actually sort of oozes out the end of the pen with the brass mechanism inside the goop, so have something handy to pull it out when this happens. DO NOT BURN YOURSELF PLEASE...
There are some alternative methods of removing the threaded brass knock mechanism that have been pointed out like putting the pen in the freezer for 10-15 minutes which shrinks the plastic and makes unscrewing easier, or slightly loosening the tip of the pen a thread or two (so there is a small gap) and then pushing the tip down on a hard surface which ejects the knock. Both options are worth a try and you may not need to use heat/flame!
You may have to remove the melted plastic from the brass mechanism, but once cooled it is easy to break off. Make sure not to mar the brass threads as this will cause you problems later.
You can now take the shiny brass mechanism and original F-701 clip and install it into the barrel. I find that it is a pretty snug fit, so you may need something grippy like a rubber glove to tighten it down. You should now have an all "metal" F-701.
To install the Space Pen refill, there is a small plastic cone inside the tip of the pen that needs to be removed. Some people prefer to shave it down or modify it, but I've found removing it to work fine. To do so, I actually just take the Fisher refill and push it into the tip in the opposite direction. It will at least loosen the plastic piece which can then be tapped out of the metal tip piece.
Replace the Zebra refill with the Fisher and you are good to go!
This hack, if done with all new parts, will run you about $20 but is a fun project that is pretty inexpensive.
I imagine the reason people do this hack is the low price, stainless steel look, and it is kind of fun. With the discontinuation of the rOtring 600, there aren't a lot of good knurled options on the market. Even the rOtring Rapid Pro is kind of "meh", particularly with the $40+ price tag. If you are simply wanting to buy a cool knurled pen on a budget to take the Fisher, you might look into the Retro 51 Hex-o-Matic. Super rOtring-esque and attainable for less than $30.