Making Polyurethane Bushes

The custom nature of the bushes and brackets makes it difficult to find the appropriate components. I made my own polyurethane bushes from a length of 1" cylinder of solid of Duro 80A material purchased from McMaster-Carr, part number 8695K497 .

I used outer bush tubes sized as per Champion's recommendations, to allow substitution of the Triumph bushes if necessary.

Inner tubes are cut to fit exactly inside the brackets as per the drawing. Use the 3/8" drill to open up the inner tube if necessary (I found trying to use the hand reamer impossible, and probably unnecessary).

Bushes are probably best made on a lathe, but I didn't have access to one. I improvised on a drill press with the following technique:

  1. Use a 1" square length of tubing as a cutting tool. Cut the tube on a bevel, allowing the leading corner to project past the others. Sharpen both edges of this corner from the inside, leaving the outside straight and square.
  2. Cut lengths of polyurethane (see drawing). Cut some extra, you need some scrap for setup.
  3. Drill center of each bush just undersize to allow a force fit onto inner bush tubes. It's not necessary to be 100% accurate, as the shaping will automatically center the bush.
  4. Press bush onto a scrap length of tube, allowing enough of the tube to project to chuck into the drill.
  5. At a speed appropriate for drilling plastic, use the sharpened square point to begin to shape the bush. Some experimentation is necessary to find the appropriate technique.
  6. Test fit the bush to the outer bush tube by removing the inner tube. The bush should just press into the outer tube by hand. Expansion when fitting the inner tube will tighten the assembly.
  7. Continue the test fitting, setting the drill depth appropriately to leave the correct shoulder on the bush. Again, the correct shoulder should result in a hand press fit into the bracket, though you can measure this just by ensuring the outer tube with two bushes is just slightly larger than the length of your inner tubes.
  8. Once the correct diameter is found, clamp the cutting tool in place, and finish all bushes, being sure the cutting tube does not move relative to the drill chuck.
  9. You can clean up the cuts by clamping a sharp chisel flat side up to the table, running the point past the inner diameter and using the sharp edge to clean up the outside lip.

Here's a staged picture. Oh, and it makes a mess.

Phil Dale suggested that the polyurethane first be frozen to help make them easier to machine. I suspect this would allow for tighter tolerances and help make the cuts more square. I'm not sure how to hold the bushes unless the tube is inserted first. Phil recommended liquid nitrogen as best, though we speculated on using dry ice or even a freezer. If anyone tries this technique, let me know how it works out.

Fitting the bushes

Insert bushes into either end of the outer bush tube. Tap a scrap bit of inner bush tube into one end, through the bush until it just contacts the other bush. Place the assembly over a suitable opening (a vise works well). Insert the inner bush tube in from the other end, protecting it as you tap it into place. It will displace the scrap tube, and seat both bushes tightly in the outer tube. A short struggle, and you should be able to wiggle the assembly into a bracket before securing with a 3/8" bolt. Clamp the tube tightly in the bracket (oh no, not this discussion again!).