Mark:
You may have done nothing wrong at all, there may have been a
problem with the countershaft. The countershaft is highly stressed, and
therefore must be hardened or it will wear out in nothing flat. I have
read of there being batches of TR6 countershafts that were not properly
hardened. In fact John Williams in his book "How to restore TR4/5/250/6"
recommends that you test any new countershaft by whacking it firmly with
the old countershaft. If you leave a dimple on the new countershaft,
scrap it and get another.
Usually it is the area under first gear that wears out first, as
the torque is highest in first, and so the loading on the countershaft
is highest at that point on the countershaft. I am betting that this is
the roller bearing that failed.
So a couple of things - Test your new countershaft to make sure
it is hardened. A machine shop could run a Rockwell hardness test on it
if you want to get formal about it. Secondly, use high quality needle
bearings - get Timken rather than some hunk of junk made under license
in India. Thirdly, see if you can upsize the needle bearing. In the case
of the TR6, there are small spacers on each end of the needle bearing
that supports first gear on the countershaft. By discarding the spacers,
and substituting a LONGER needle bearing that does not need the spacers,
you can increase the bearing area by 30% and so extend the lifetime and
strength of the gearbox. This requires no machining, all you need to do
is go to a bearing supplier and show them the parts. They can do some
measurements and give you the proper upsized bearing. If the Jag tranny
does not have spacers, then you can have the gear cluster machined to
accommodate a longer bearing.
When you disassemble the tranny, keep the old countershaft and
have the hardness checked on it, and if it is not properly hardened, you
will have your answer as to what went wrong.
Cheers,
Vance
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net] On
Behalf Of Mark
Sent: May 26, 2006 8:32 AM
To: 6pack
Subject: chewed up countershaft
I'm trying to learn from whatever mistake I made a couple of years ago
when I rebuilt my transmission (76 with J-type).
Everything was good for the last two years of driving, but at the end
of last season, I was getting some very rough shifting. So I parked
it, and starting digging into the problem. First issue, one or more of
the roller bearings had disintegrated. There were several of the
little cylindrical bearings stuck to the drain plug magnet.
So I pulled the transmission and finally got it opened up last night.
I pulled out the countershaft rod, which was replaced brand new a
couple of years ago, and it has the same chewed away condition the old
one had developed, as if it had been turned on a lathe, except of
course not that smooth or even. There is no damage thankfully to the
main gear cluster, center bearings or synchros.
So as I do this again, does anyone have a suggestion as to what I
might have done wrong that would have caused the bearing to explode
and the countershaft to get chewed up? I do only normal non-aggressive
driving - no autocross, no racing, no jackrabbit starts.
--
Mark
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