November 2018
solid spacer, between the pinion bear-
ings. These minor complications are
easily to overcome but I will not deal
with them here because these ratios
really only suit race cars.
Before starting a conversion, I suggest
checking the mesh with “bearing blue”.
Hopefully it will indicate that no adjust-
ments need to be made. (Dismantling
and reassembly should not alter the
mesh if everything is put back together
exactly the same way.) The two bearing
caps and crown-wheel assembly should
then be removed. The old flange holes
must be filled. I thread the holes and
then fill them with bits of threaded alu-
minium rod and Araldite or “JB Weld”.
The gasket face then has to be ma-
chined back 3mm in a lathe. I have a
mandrel designed for mounting the
housing on a lathe. Before doing this,
the pinion can be masked off to prevent
the ingress of swarf, although I always
remove the pinion, so a new seal can be
fitted during final assembly. New holes
must then be drilled to match the TC
banjo studs. I used to do it the hard way
but now I have a device (courtesy of
Brian Jeffrey) to position these new
holes. Club members are welcome to
borrow these tools.
The 10-spline Morris spider gears have
to be replaced with 6-spline spider
gears so that the TC axles will still fit.
These spider gears can be harvested
from a TC, Y-Type, Morris 10 or Morris
8/40 diff. While the spider gears are out,
the sides of the carrier will probably
need slight grinding out, so the TC axles
will easily fit through. I use a stone in a
hand drill to do this job.
TCs have four planetary
gears but only two can
be used on the single
Morris cross-shaft. Se-
lect the two gears which
are “loosest” on the
shaft and linish the shaft
if necessary to ensure they have detect-
able clearance when dry. The Morris
spider gears have bronze thrust wash-
ers. Discard the small dished washers
but retain the large flat washers. Some-
times these large washers need a rub
on emery paper, to thin them down, so
that the spider gears will rotate freely.
It is more difficult to do an A-Series con-
version to fit a Y-Type because they
have a much smaller diff flange. When
machining the gasket flange, the hous-
ing and caps must be substantially re-
duced in diameter and the Y-Type banjo
must be ground out heavily, so that the
diff will fit into it. The bolt pattern is also
significantly smaller. I once encountered
the same issues when I converted an A-
Series diff to fit a J-Type.
Use a good quality pin to secure the
cross shaft. I use the shank of an
“Unbreako” 4mm bolt, and lock it in well
by using a centre punch around the
hole. Tightening torques are 65 ft/lbs for
the bearing caps and 140 ft/lbs for the
pinion nut (assuming a solid spacer).
After final assembly, check the crown-
wheel/pinion mesh with “bearing blue”.
There is no air vent on the TC banjo, or
on the A-Series diff, necessitating the
drilling of a 3mm hole in the top of the
banjo, on the right hand side, near the
brake hose connector. Y-Types have
such a breather as standard. Use sili-
con sealant instead of a gasket when
installing the diff. Use modern “HYPOID”
90 gear oil in the diff.
(Updated from Bob’s late 1970s MGCC
Magazine article.)
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