>> My '76 TR6 has a non-functional O/D and the gearbox slips out of 3rd
>> gear when decellerating from high RPM's (around 3500-4000).
>
>I don't have an overdrive, but my '72 TR6 also pops out of 3rd on
>deceleration (more easily than Mike's, though--it doesn't take as much RPM
>to do it. Rolling is just about enough...)
>
>> 1) Does the 3rd gear problem require a gearbox rebuild, or is there a
>> quickie repair to solve that?
>
>And if it does require a rebuild, what specifically am I looking for? My
>box has been rebuilt twice in the past 3k miles, and this behavior began
>after the second rebuild. The parts are almost entirely shiny-new.
It requires some work at least, perhaps not a full rebuild. There
are two things that alone or in combination cause jumping out of gear:
1) weak detent spring on the 3rd/4th shift fork. This _probably_ wouldn't
cause it alone unless it was very weak.
2) the "dog teeth" on the 3rd/4th selector collar(?) or the 3rd gear/synchro
are worn at a slight angle on the overrun side. The teeth are under the
full torque load of the engine, and angled wear will cause the dog teeth to
push each other away, eventually popping out of gear. On overrun, there's
less torque, but you still have full engine braking torque. Also, every
time it pops out, it can increase the amount of angled wear.
Vibration can play a part too. I suggest at a minimum replacing the detent
spring or increasing it's force (stretch, shim, whatever). You should really
check all the visible dogteeth in that gear for wear, and probably take it
apart to check and maybe replace the sleeve and/or gear/dogteeth.
A worn selector fork is theoretically possible too.
I dealt with this in my first car, a '65 Sprite. It popped out of 4th all the
time under acceleration. Finally I pulled the engine and trans in college
and found the slight angle on the input shaft dogteeth. Since I didn't have
a spare mainshaft, I just reversed the 3rd/4th collar, moving the angled wear
on the collar from on-power in 4th to off-power in 3rd (on a Sprite they were
reversible). This mostly worked, though max torque going up a hill would
still pop it, but I could mostly cruise in 4th on the highway for a while.
It deteriorated again from occasional pops, though - the input shaft really
had needed replacing.
I got quite used to cruising in 3rd at 55 on the highway, at ~5500 RPM, on
the edge of yellow (1098cc big-bearing A-series)...
--
Randell Jesup, Scala US R&D, Ex-Commodore-Amiga Engineer class of '94
Randell.Jesup@scala.com
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