Wow I've missed a great discussion. A bushing with a spiral groove that starts at the center with an annular channel could work but the spring eye and the bushing needs to be much larger to support a thicker bushing. Since the bolts are in shear, they're very strong where the weak point would be on the nut side of the pin and fails IMO from over tightening.
Long ago and before engineering, I had changed out many king pins on heavy trucks (before two major back surgeries) which were fed grease from each side of the pin and knuckle. The bushings were a spiral design and worked well. Since sealing the pin on one side for our rigs is not possible, the bolt must be fed from the center but must have a suitable diameter and bushing thickness with a feed channel from the center and outward to do this. Currently this is not done with no area to retain grease where suspension motion will quickly push out the grease on the flat bushings.
All we can do today is give the pins a shot or two of grease before each trip.
Jim
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