I was originally going to just send this as a PM to Optimist_Prime, but since other people seem interested, and because the technique itself is pretty useful, I figured a general thread would be good. I wanted to add waist rotation to a Downshift mod. Since the figure's no longer using his Powerlinx ability, I've fused the crotch pieces together into one lump (in this case, with Cyb Hot Shot's hip joints too, but that's another story). Now I just need to add the swivel joint for the waist. Unfortunately I didn't think to take WiP pics as I did it, but I think the finished photos, in conjunction with my explanation, should give a good idea of the process. Using a cylindrical burr in my rotary tool, I grind out a hole in the top of the crotch piece, at the point where the four pieces have been glued together. The hole is to be big enough to accommodate a decent thickness of styrene tube (I've selected a piece with an external diameter of 5.3mm and trimmed off a couple of centimetres to serve as my post). Using a good superglue, I cement the post into the hole. Once set, I fire up my hot glue gun, and pump a little glue down into the channel of the tube. This isn't entirely necessary, but I find it's a handy (and cheap) trick to reinforce hollow components, just in case. I hold the two halves of the torso together (not bothering to rescrew them), and again using the cylindrical burr, I grind out a corresponding hole in the bottom of the waist to accept the tubing. I test the fit of the post, and trim the shaft to length, and make sure any excess glue is cleared off. Finally, I need to add a lip to the shaft so that the figure doesn't do a Movie Jazz and keep falling in half. There are two main ways of doing this; one is to use a fairly thick sheet of styrene, trim off a small section a little wider than the tube, glue it to the end and file it into a circle. I choose to use a thin piece of styrene strip; in this case 2mm wide by 1mm deep. I cut off a short length, and glue one end to the end of the tube with polystyrene cement. I feed the strip round the circumference of the tube, gluing a bit, holding it place with a small clamp and letting it set before moving on to the next area. The strip will naturally try to straighten till it's set, and it's extremely difficult to hold the whole strip tight to the tube while it sets. Once it has set, I test fit it, and trim off any obvious obstructions. I find the swivel's a little loose, so I remedy this with a few successive coats of liquid poly cement. This not only makes the post thicker, but it also has the added advantage of strengthening the bonds of the styrene. Then the torso can be screwed back together round the new post!