There are about a dozen things that can cause the popping out of third
gear, but 90% are due to two things:
1. Top hat bushing broken at the flange, or bushing endplay too great.
This can often be diagnosed by taking the top off the trans, moving the
gears so it is in third gear, grasping input shaft in one hand and
output in the other, and counter-rotating them. If you see the third
gear move axially on the mainshaft, the endplay is too great. (Actually,
this can all be done with the trans in the car, but you have to be a
contortionist).
The fix requires complete disassembly of the mainshaft, replacement of
bushings, and especially proper re-shimming, which means getting that
%&(*## square snapring off. I have resorted to treating it as a
throwaway item, destroying it to get it off, an putting a new $0.95 ring
on.
2. Detent springs in the transmission top, the ones that go into the
notches on the shifter shaft, are too weak. There is a factory spec for
this on the TR4 and on the TR6. I believe the spec is 32 lbs for the
TR6. Go to the hardware store, buy a $5 fish scale, and with the trans
top held upside down in a vise, see how many pounds force are required
to move the shaft off the detent.
I just went through the detent problem on my race car, with brand new
third gears and shifting collars in place ($$$$). After having the trans
in and out of the car three times, I measured pull-out force, found it
to be only 12 lb, I ordered new springs from Moss, put them in, and
found them to be just as weak as the old springs. Then I went through
all the trans tops I had, found the strongest spring, went to the
hardware store and bought a little spring that would just slip inside,
reassembled, and got about 28 lbs pull out force. Thanks to my fellow
racers who suggested this fix -- this was not an original idea.
BUT -- that solved the problem.
Good luck!
--
TR6 -- 29 and still running
TR4 -- 39 and still racing
uncle jack -- 63 and still learning
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