[TR] overdrive gearbox switches

[Randall]

The TR3 workshop manual has detailed instructions for selecting those shims.
Basically you select the gear and screw the switch in until it just closes,
then measure the gap between the switch body & the cover.  Subtract .040"
from the measured gap to get the required shim thickness.  So for example,
if you measured .090", you need to assemble a shim pack for .050".  No
tolerance is given, but I would guess that +/- .010" would be OK.

The most important aspect is obviously that the switches disable the OD in
reverse; but the book talks about the OD engagement needing to be after the
synchros are meshed as well.  

Keeping the same pack would probably work, but with all the variations in
parts, I would want to check anyway.

-- Randall