439
Gearbox
trouble with a Factory MGBGTV8 box
Mike Clemas posted a query on the V8BB saying "just been
under my Factory V8 as it's had a very slight oil leak from
the gearbox for a while. Turns out it's got a hairline crack
in the casing not far from the drain plug. Has anyone had any
success welding cracks, or is the end of the gearbox in sight?
I know the LT77 is a better option (and I have one sat under
the bench) but I really like the 4speed gearbox with overdrive,
just a little more interesting to drive in my opinion."
Nick Gray responded with help. (Jan 12)
External view of the welded crack in the gearbox casing.
Nick Gray responded saying "I have a gearbox
casing that has been welded in what sounds like the same position.
I understand this is a common crack caused by shock loading
the reverse gear (putting the car in reverse before it has lost
all forward momentum) causing the reverse gear shaft support
to fracture at the base. I bought the gearbox for breaking which
needed a lay shaft, but understand from the previous owner it
been working well. The downside is that unless the welded repair
is spot on you suffer a noisy reverse gear due to slight shaft
misalignment. So yes, welding is possible but it is a tricky
job to get exactly right - notwithstanding that welding castings
is tricky in itself as thermal stresses can crack the weld as
it cools.
Graeme
Don mentioned the topic had been raised in the popular series
of MGBGTV8 rebuild reports from Barrie Jones in 2010/2011 published
on the V8 website. See Report
15
A few teeth missing from first gear -
the probable result of V8 torque and too many wheels spins from
stationary.
Nick Gray offered to provide some
photos which would help illustrate the welded repair and the
reverse gear shaft support and several weeks later he sent in
a set of photos saying "apologies for the late reply, I
haven't had much time to get out to the workshop this month
and since it is not heated I have also lacked motivation to
do so! So with a brighter and warmer day today, I |
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Inevitably the lay shaft in the second
gearbox had also been damaged beyond repair - a pity as these
are becoming next to impossible to replace.
have taken the following pictures of the remains of the gearbox
casing which clearly show the weld repairing the reverse shaft
support from the outside of the box.
I
have also taken a few photos showing the interior of the gearbox
and you can see the rather messy weld on the left hand side
of the support where the bracket has been reattached to the
case. You can also make out a bit of the weld repairing the
crack in the casing the reverse gear support caused when it
broke. I think this was a fairly poor repair - the bracket is
a few degrees off the perpendicular to the case and reverse
shaft that ran through it which probably explains why the chap
who sold it to me had opted to switch to a five speed conversion
whilst happy to warrant that the gearbox "worked".
However the box did have a perfect layshaft, first gear and
internals which, when combined with the perfect casing of another
broken gearbox I found, has given me a very good spare which
is in my V8 at the moment. The second box had damage to the
first gear and layshaft - a typical failure of these boxes I
understand. I have added some photos of the smashed lay gear
and first gear from that box which may be of interest for another
article - although the damage was much more extensive due to
the box digesting the broken gear teeth.
The gearbox I have had rebuilt from the remains is a delight
to drive - my only disappointment is a slight whine, the cause
of which I have had great difficulty tracking down - the rebuilt
box had all bearings, synchro rings and other parts replaced
with new parts. Having now spoken with a few people I understand
that the gears in any box 'get used to one another' and the
meshing faces of the gears and loaded parts of the shafts undergo
a bit of work hardening. Such is the tolerance of gearboxes
of this era, that when parts from two different boxes are combined
it takes a while for the meshing faces of the gears to get 'used
to one another'. I was initially very sceptical of this explanation
but after 2,000 miles with the new box the whine is slowly fading
- or perhaps I am becoming used to it! I have felt confident
to give the rebuilt box a fairly harsh time and it has proved
itself - the most noticeable improvement is the accuracy and
feel of the gear shift which is much tighter.
My
next task is to refurbish the original gearbox removed from
the car which had a very noisy layshaft and I expect to find
worn bearings when I get around to stripping it in the spring.
It has covered 80,000 odd miles from the history I have of the
car, so has probably had a good innings for a V8 box? I will
then keep it as a spare - especially given the time and expense
it took to find suitable bits to knock together the 'new' gearbox!
See our illustrated note. More |
