My kids were playing and broke the head off of one of their Barbies. I searched the web and ran across what the original pin looked like for inspiration (Pic attached from here: http://babes-in-toyland.blogspot.com/2013/10/How2-behead-babs-well.html). I noticed that the bar that went through the loop was broken, so I used a dremel to clean out the neck hole a bit and designed this peg to slide down into it. I printed the part in 2 halves and glued them together and then glued the peg into the Barbies neck. Once the glue was dry, the head popped right on and was able to maintain full turning motion.
I recommend test fitting everything before you glue the pin in. You can pop the peg out of the Barbie head using a small flathead screwdriver; just don't let it fall all the way into the head.
The parts were run through NetFab online before being uploaded.