You had it right the first time. The original shim (pack) is always the
place to start and almost always ends up being correct, assuming that all
other parameters are met, (preload, backlash, etc. depending upon the type
of R&P set up you are dealing with). At times, I have passed up the
opportunity to use an NOS R&P set in favor of undamaged used gears just
because of the ease and speed with which you can install a (used) set with
a known baseline.
The small section contact patch you were missing would become full under
deflection due to load. I am always amazed by the amount of forces on and
abuse handled by ring and pinion sets. If you have ever seen a drag racer
doing a wheelstand, what you are really looking at is the pinion climbing
the ring gear and the entire weight of the front of the car being supported
by that tiny contact area you observed.
Measure the old shim and get a new one.
Regards,
Jeff
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