Well, if you really don't want to invest the time to tear the box down and
replace the bearings, check out the layshaft and such, you can get a rough
idea of the condition by poking around with the top cover off. But first,
see if the effort to move the shift lever is about right. Adjusting the
tension on the control rod detents is quite a bit easier with the box on
the bench.
Check the condition of the synchro rings. The metal shouldn't be too dull
and corroded. The teeth on the gears and the yellow metal rings should be
well defined. If the edges are too rounded, the box may be quite willing to
jump out of gear, regardless of the detent tension. Using a small screwdriver
or two, push the synchro rings onto the gear bevels. If the rings are too
worn, then they go too far and rather than a smooth transition of speed, you
get a sudden lurch when the face of the ring hits the gear, rather than the
tapered surfaces mating.
And of course, wiggle the input shaft to check on bearing play, same with the
tailshaft. Then when you are satisfied that the gearbox will probably work,
go ahead and order in some bearings, new synchros and such and put the time
into freshening up the box while it is on the bench, so you don't end up under
your TR6 on the side of Big Cottonwood Canyon road in a blizzard, wondering why
you ignored those noises and took the car anyway, instead of the bus.
mjb.
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