MG Car Club of South Australia
TeChnical TopiC
by Bob Schapel
S
TC DIFFERENTIALS
ince 1976 I have converted forty five
BMC A-Series diffs to fit TCs. It is
not a difficult job to modify them so they
bolt straight in, and there are lots of ra-
tios to choose from.
In 1976 I was looking for a simple way to
get “taller” gearing for my TC Special, so
I wrote to the MG club in UK and The
New England T-Register in USA, to see
if they had a solution. The UK reply
stated that Ford Anglia, Escort, Cortina
and Capri diffs were their solution. Fit-
ting those diffs did not appeal to me be-
cause they needed specially made half-
shafts (as was the case for the Holden
option, which was common here). I got
two replies from USA. The first sug-
gested that I fit a complete MGA or MGB
rear axle but the second (from a chap
named Chip Old) turned out to be what I
was looking for. Chip explained that it
was not a common conversion, but
someone had used a “Spridget” (A-
Series) diff in a TC. His letter inferred
that standard TC axles could be used.
He outlined the process but apologised
for not knowing the details. Morris Major
and Spridget diffs are interchangeable,
so I went to a wrecker and bought a 3.7
ratio Morris diff to play with. Reposition-
ing the holes and skimming 3mm off the
gasket flange face, allowed it to bolt
straight into a TC banjo. A bit of experi-
mentation showed that 6-spline TC spi-
der gears could be easily fitted in place
of the 10 spline gears, allowing the origi-
nal TC axles to be used. Within three
weeks I had that diff in my TC special,
and two months later, I also had a 4.55
Morris Minor diff in my white road TC.
Now, after forty two years, the same
36
4.55 is still in my road TC and I still fit
that same 3.7 (to suit the long straight)
when I race the supercharged TC spe-
cial at Phillip Island.
I wrote a “Technical Topic” in our maga-
zine in late 1976 explaining how to do
the conversion. However, there seemed
to be a strong demand for ready-to-fit
diffs, so I converted several for fellow
TC owners. I did many more to sell
through Bob Bazzica’s “MG Sales” until
the late 1980s. I punched my initials on
the bottom of each housing, and kept a
list of the crown-wheel/pinion match
numbers, in case a faulty unit was re-
turned (although none were). I also con-
verted a few as alternative ratios for the
two “Schapelli Racing” specials.
The first step is to acquire an A-Series
diff, in good condition and with the de-
sired ratio. It must have an aluminium
housing. (Very few have cast iron hous-
ings.) There are eight A-Series ratios
available. Very early Morris Minors have
a 5.375 ratio (Perhaps suitable for cars
used in VERY hilly regions or raced on
VERY tight tracks). Austin A30s have a
5.125 ratio (Same ratio as a Standard
TC). A35 vans have a 4.875 ratio (Same
ratio as a TA ratio, but pretty rare). Mor-
ris Minor 1000s have a 4.55 ratio (Ideal
ratio for a TC, especially with 16”
wheels or over-bored engine). The
“taller” ratios, 4.2, 3.9 and 3.7 were fit-
ted to Morris Majors, Riley/Wolseley
1500s, MG Midgets and Sprites. A com-
pany in Germany also makes 3.5 gear
sets to suit. These “taller” ratio diffs
have slightly different housings which
need extra work. Their strengthening
webs need trimming and an oil filler/
level plug has to be fitted. They often
have a collapsible spacer, instead of a