Once again a task that I didn't think would take too long required much more time and effort than was expected. In this case it was the spherical metal grommets used to carry the throttle, mixture, and prop cables thru the firewall.
The installation began with drilling out the aluminum spheres to 11/32 to allow Van's cables to pass thru but still be gripped by the eyeball. I began with a 1/8" bit and increased by approximately 1/16" until finishing with th 11/32" bit. I thought about stopping at 21/64" but the fit seemed just a little too tight. I used a drill press and vise but even so, the holes are not perfectly centered. They are, however, close enough and not really noticeale.
Installing the grommets in the firewall was another challenge. I bought a carbide hole saw normally used for cutting 3/4" conduit holes in metal boxes including stainless. The 3/4" bit actually cuts a hole 1-1/8" in diameter which is required for the grommets. Cutting a clean hole in the stainless firewall is not easy. I used the hole saw both with a backing plate and without. My first attempt didn't go well. The cutter grabbed and cut too deep on one side. I had to finish the hole with a Dremel and a rotary cutting bit (time consuming and difficult). Anway, I was eventually able to clean the hole up. The second and third attempts went much better. The trick is to use light and very even pressure. You want to be sure the saw is cutting evenly around the circumference.
Finally, I marked the six screw holes for each grommet and drilled to #30. After deburring, I installed the backing plate and then the spheres with clamping plates temporarily.
