If memory serves me right, the rod ends I purchased were part number 2458K111 fom
http://www.mcmaster.com
The exact specs really aren't that important. Anything even close to this will work fine. The only thing is that the rod you slip through it must be the same diameter as the opening of the rod end.
The main piece that I had machined for me was the rod that sticks up through the ball joint rod end. I don't have a good picture of it yet, as the only one I have is mounted in the prototype skull at the moment, but here is a drawing I made and a description:
I don't have the exact dimentions, but as long as you make it to fit whatever coupler you are using and whatever rod-end you are using it will work fine. The actual dimentions should be something close, but are not that important.
The rod starts out thicker, somewhere around 1/4 20 bolt size at the bottom with threads cut into it so that it will fit in the threaded rod coupler I bought at the hardware store. The shaft extends up for about 3" or so and is then shaved down with a flare so as not to restrict movement as it goes into the rod end, but it keeps the rod end from moving any lower onto the shaft.
Above the rod end I have a thin spacer made of nylon. At that point, the shaft is threaded again. The threads match the two nuts that are attached here.
The portion of the shaft going through the rod end is not threaded. It probably wouldn't make any difference, however.
The first nut is tightened down over the spacer and holds the rod end tightly in place. Then a small aluminum arm is placed on the shaft with another nut on top to hold it in place. This arm sticks out towards the back of the skull and is for the servo which controls Rotation (left right movement) to attach to.
The very top of this rod has a small hole drilled in it in the center and it is threaded (inside threads) for a 4/40 screw. This screw can be seen in the picture in my first post. It holds the two armatures from the other two servos in place. They control the nod movement (up and Down) and the tilt movement of the skull. So there are your 3 axis'
I will post another picture of the skull mech including the jaw servo shortly.