Hi again,
I'm just throwing out some ideas here, not really certain what's happening.
Regarding shims, does anyone know how to determine how many should be used? I don't recall ever seeing anything about measuring for starter shim installation in any shop manual.
If your car had 4 shims in there, that seems like a lot. Maybe it's typical with the bull nose starters, I'm just not all that familiar with them. Are there 4 shims in there now? If the starter gear over-engages, maybe because there aren't enough shims, it might not disengage properly and the starter might be damaged after the engine starts. Then, when the car is shut down, the starter disengages as it should, but the damage is done.
Regarding ring gears, the teeth are chamfered on one side of the ring gear. I'm not sure if this is the case with the pressed on ring gear, but it is with all the bolt-on ones I've seen. There are correct and incorrect orientation of the ring gear.
I believe your bull nose starter engages by pushing the starter gear rearward, so the chamfer on the ring gear teeth sould be facing the front of the car, I'm guessing. Also, shimming would move the starter away from the ring gear, reducing the amount of mesh.
The later starter has the gear sticking out to the rear, and it's pulled toward the front of the car to engage. In this setup, the chamfer on the ring gear teeth should face the rear of the car. (This is the setup on my TR4.) Also in this case, adding shims moves the starter gear closer to the ring gear and increase gear mesh when it'sengaged.
Changing from one type of starter to the other, or swapping aroudn flywheels, also usually means orienting the ring gear properly to match the starter. However, I gotta add that I presently have a modern gear reduction starter on my TR4, which engages by pushing the gear rearward, just like the TR3 bull nose starter. The ring gear on my car is "incorrectly oriented", because it's still set up for the pull-type Lucas starter, with the chamfer on the trailing edge. But, the vendor who sold me the starter and other users have told me it doesn't seem to matter with these modern starters.
One other thought, the bolt-on ring gear is held on by 6 bolts, if I recall correctly. And, there are locking tab washers under those bolt heads, that should be bent to keep the bolt from backing out. If the tab washers were omitted, one of the bolts might back out and cause some interference. For that matter, if one of the locking tabs were improperly installed, it might cause some interference.