You wrote:
>What are the probable causes for the transmission in a 73 TR6 popping
>out of third gear? Does this indicate the layshaft bearings (and
>probably the shaft itself) are shot?
My experience with the TR6 gearbox concerning the layshaft bearings
has not had any relationship to popping out of 3rd gear. The
layshaft bearings will typically (almost universally, in fact) fail
at the 1st gear end of the layshaft, and this manifests itself by
making crunching noises and by making it difficult to engage 1st gear
or reverse when sitting still and shifting between 1st and 2nd
becomes difficult as well. I've experienced this exact scenario
TWICE with my TR6 gearbox. In neither instance did it cause the 'box
to jump out of third, and in BOTH instances the rear (1st gear end)
layshaft bearing was completely destroyed. And of course, the
layshaft was pretty well toast as was the rear thrust washer.
I drove on a spare gearbox for a while that jumped out of third, but
it would only do it while coasting down in third. If I was on the
throttle, or on an even throttle, it would stay put. But the minute
I lifted to let the car coast down in third, the stick would pop out.
I'm going to assume that yours behaves the same way. Perhaps the
most likely cause might be a worn 3rd gear bushing which is giving
too much end float on 3rd gear. Along with that, perhaps, is the
specter of the infamous 2nd gear 'top hat' bush, the brim of which
frequently breaks free of the remainder of the bush, possibly as a
result of too much end float between 2nd and 3rd gear. It could also
be that the dog teeth on the third gear are badly worn or damaged, so
there's not as much tooth for the outer synchro sleeve to hang onto.
I would also point out the detent ball and spring for the 3rd/4th
selector shaft in the top cover. A weak spring under that ball might
allow the fork to slip forward, disengaging third gear, however if
this were the problem I'd expect it to act up in fourth gear as well.
There are also 3 detent balls within the synchro hub itself with
springs behind them and I suppose if the springs get weak enough, it
will no longer hold the outer synchro hub in position and it could
snap back into neutral. Again, I'd expect this to cause problems in
EITHER 3rd or 4th.
Either way, you're definitely due for a complete and thorough
rebuild. I just finished rebuilding mine last fall. Although this
is the second time I've rebuilt my gearbox (the first rebuild lasted
13 years) I don't by any means consider myself an expert in the area
I'm learning. The above suggestions are my best guess based on what
relatively littel experience I have and I'll be interested to see
what those that are far more knowledgeable in this area have to say
about your symptom.
Best of luck!
--
Pete Chadwell
1973 TR6
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