The trick is that Tornado refills have this odd little cap on top to make them retractable.
Step 1: Remove the Tornado refill top any way you can. (I just pulled it off with pliers)
Step 2: Cut the G2 refll to 3.3/8" (8.5852cm) (I came to these measurements through trial and error).
Step 3: Tape the Tornado refill top to the G2 refill and voila, works perfectly. The taping doesn't even have to be that precise or careful. I just pulled off some Scotch tape and done (please note, that I've found that it works better if you put the refill into the body of the pen first to make sure that the retracting mechanism latches to the refill properly) :)
- Hacking a Pilot Hi-Tec-C Refill into a Retro 51 (uses Pilot Hi-Tec-C)
- Retro 51 Tornado refill hack (uses Pilot Precise V5 RT)