REPEGBy Sicxer
The RE stands for retractable, and the R and P is kind of a short form for RSVP which is used in this mod. The G refers to the G2 used in the mod. It should be pronounced as a word.
Pens needed:
1 Pentel RSVP *with empty ink tube*
1 Pilot G2 *with half empty ink tube*
1) Preparing the RSVP:
Take out the cap and unscrew the back cover of the barrel, you don't need these. Take out the grip. Cut off the metal writing tip of the empty ink tube so you have a long clear plastic tube.

2) Preparing the G2:
Break off the clip of the G2 and file it down till it's quite smooth. Remove the grip and cut it in half. Fit the smooth half of the grip onto the back of the G2, at the clicker part. Note: in the picture the ink tube is full. You should use a less then half empty ink tube.

3) Cutting the writing end of the pen:
Refer to the picture. Place the top half of the G2 grip as close as possible to the tip of the G2 front barrel, but not too close that you can't write with it. The RSVP grip goes right below it. The base of the RSVP grip should cover the threaded end of the RSVP barrel. Cut the G2 front barrel as shown in the picture. After cutting, the G2 barrel and RSVP barrel should touch each other and be covered by the RSVP grip.

4) Cutting the clicking end of the pen:
Refer to the picture. You will need to cut the tip of the RSVP barrel and the back barrel of the G2. Cut the RSVP barrel roughly 2cm from the tip. Cut the G2 barrel roughly 2.2cm from the screw threads end. Smoothen it out and fit the G2 onto the RSVP like you would with a MMX. The newly cut tip of the RSVP should align as close as possible to the groove on the inside of the G2 barrel, with the G2 being able to fully fit onto the RSVP. (see picture) Slowly file off the barrel if they do not align, until it fits nicely. The G2 barrel will not fit tightly onto the RSVP, so you will need to put an insert. (For size of insert, refer to part 7 below)

5) Ink tube cutting:
Now you need to cut the ink tubes to size to fit in properly. If you'll notice, the G2 ink tube gets stuck about 1/3 into the barrel (pushed in from the threaded end of the RSVP barrel) so you'll have to cut think ink tube. Cut about 3cm off the back of the G2 ink tube (this is why I said half full ink tube), push out the tube cap with the RSVP ink tube, and fit it onto the shortened G2 ink tube.

6) Assembly:
Now you'll need to assemble the pen before you can measure the required length needed to be cut off from the empty RSVP ink tube. First, superglue the RSVP grip onto the threaded end of the RSVP barrel. Don't use too much glue or it will leak out when you squeeze the grip on and create a mess. Once the glue dries, put the shortened G2 ink tube into the barrel, and then insert the G2 front barrel into the RSVP grip. (see picture)
Put the RSVP ink tube into the RSVP barrel, going in by the clicker side, and then put the G2 back barrel onto it. Make sure the RSVP ink tube goes into the clicker part of the G2. You'll see that the ink tube is too long, such that the G2 back barrel cannot reach the RSVP barrel. Slowly start cutting off the RSVP ink tube, shortening it bit by bit till it finally fits just nice, allowing the G2 barrel to fit onto the RSVP barrel, and it must be able to click nicely. Note, don't cut off too much of the ink tube, or the clicker will be loose, causing the pen to rattle and not click properly. The exploded view of the assembly is below.

7) Inserts:
To be able to fit the G2 barrel tightly onto the RSVP barrel, you need an insert. The insert has to be on normal paper, and has to be only 1 sheet thick. Thicker and the G2 won't fit. Wrap it around the shortened RSVP tip and push the G2 barrel on.
These are the inserts I made:
The insert that fits into the RSVP barrel:

The insert that fits into the G2 barrel:

~//~
There, it's done. The finished product:
Measured against ruler:

Balance, in comparison with RSVP MX:

Presentation video:
ZippyvideosRapidshareStats:
Length: 20.2cm
Weight: 19 grams (RSVP MX: 18 grams)
COP: 10cm from clicking end, 10.2cm from writing end
Cost: Less then US$2
Pro:
- 2 common pens needed only. Cheap and easily accesible.
- Equal sized barrel along the whole length, no caps or bumps to affect spinning.
- Long with COP nearly at center.
- Durable, with 'shock absorbing' grips. No signs of cracks after 1 week of testing.
- Refillable with G2 ink.
- Grips on both sides. Equal feel on both sides of the pen.
- Great for infinity's and sonics.
Cons:
- Difficult to make. Requires precision cutting. Time consuming.
- Insert can only go in a certain length into the pen, leaving empty gaps in between.
- Too long and heavy for newcomers to learn fundamental tricks on.
- Requires nearly empty ink tubes. G2 ink can only be refilled when on low ink.
- It's a gel pen. This may cause leakage under excessive dropping.
Things to consider:
Originality: It's different from all other retractable mods I've seen. And cheaper then most too.
Appearance: Well, since the insert does not cover everything, you can see the parts of the pen joining together, which gives it a 'broken up' look. How you like this look, I don't know.
Spinnability: Good COP, good weigth and length and smooth feel to the pen. See video for more info.
Durability: See last part of video. Dropped from height of about 1.5m.
Creativity: Using the ink tube to push another ink tube. Holding the pen together with the grip. Method of joining the G2 to RSVP.
Overall: Is this mod worth your money? Dude, it only uses 2 pens to make a retractable mod! It makes full use of the G2, and you will only have a RSVP cap leftover, which you can reuse on a double capped RSVP, or StriderBlade's cyborg above.