To address a couple of questions posted re: availability of the bushing for others, diameters etc.
We are currently looking into having these bushings manufactured. Luke at TreeStuff.com will have them if there is interest and we go through with this.
The outside diameter of the bushing itself is the same as the original set-up, e.g., 1/2". Therefore Kevin's design specifications are maintained. The inside diameter is 5/16", oversized a couple thousands to facilitate easy insertion of the pin. This then is the same as the original so the inside hole accommodates the removable pin that is available through Luke.
Tolerances of my bushing are close to be exact, there there is no slop in the bushing/pin interface. Does it rotate? I don't know the answer to this questions, however with a sliding load on it, it may. My testing, albeit limited because I don't climb every day like many of you, is limited.
I don't want to imply the pin itself is unsafe, especially if one is using a pin with a known rated shear force. Nevertheless, if there is going to be a failure, it would be the result of the pin bending. The bushing distributes the load in such a way that one is dealing with shear forces that would have to fail to be catastrophic, highly unlikely. The other thing it does is maintain the spacing between the two points as originally designed.
Eric