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T SERIES GEAR RATIOS
Part I
by Ben Prince
In my opinion, the TD and TF are geared too low. In the
typical TD, the engine is turning about 4200 rpm at 60 mph in
fourth. With the long piston stroke at this relatively high RPM,
it sounds like the engine is about to become unglued at highway
speeds. And it probably is!
The first thing that comes to mind for the low gearing
problem is to install larger tires. This is a step in the right
direction, but to have much of an effect the tires would have to
be disproportionately large -- a particularly distasteful
solution unless you like T series MGs looking like street rods.
Another approach is to buy one of the kits to install a
Datsun or British Ford five-speed transmission, but it seems to
me that this is too great a transgression of authenticity to be
acceptable.
In my opinion, a better approach is to change the rear end
to higher gearing. The standard TDs and TFs came with 5.125 and
4.875 rear ends, respectively. Changing to either a 4.555 or a
4.3 ratio results in an improvement in lowering RPM at 60 mph.
The 4.555 was an original option for the TD/TF. According to the
Moss Motors catalog, the 4.555 was also found in the MG ZA/ZB and
in some MGAs. It can be argued that the 4.555 is the optimum
choice for those cars with the 1250 cc engine. On the other
hand, the 4.3 ratio seems ideal for the TF with the 1500 cc
engine.
Although new 4.555s have recently become available, they are
quite expensive. And finding a used 4.555 is a difficult
proposition, the only real salvage-yard-option being the 4.3.
The 4.3 is found in most MGAs and relatively speaking, is readily
available. The 4.3 will serve both the 1250 cc and 1500 cc cars
very well, certainly with less stress on the engine than the
original standard installation. Switching to 4.3 allows the
TD/TF engine to turn a comfortable 3500 rpm at 60 mph.
The downside of the MGA 4.3 is that there will be a
degradation in acceleration and hill climbing ability in fourth
gear. But third gear will now be higher than it was before the
rear end conversion, with third becoming more useful and down
shifting more practical. In any event, the reduction in fourth
gear performance seems acceptable -- in my opinion offset many
times over by the improvement in engine life and drive-ability.
Unlike the TD/TF, the TC is not geared all that low because
of the larger wheels. The TC 5.125 gearing with the 19" wheels
is equivalent to a TD/TF ratio of 4.66 with 15" wheels -- perhaps
still a little low, but not at all bad. I am told that MGA
conversions are not possible with the TC, but Sprite/Midget 3.9
conversions are. The 3.9 switch would probably make the gearing
too high, however, particularly with the 19 inch TC wheels. It
appears that leaving the TC alone is the best bet.
There are two approaches for changing the TD/TF to the MGA
4.3. The first is to totally replace the TD/TF rear end with a
complete MGA unit. This is a fairly easy process that is
primarily a bolt-on exercise. The spring perches have to be
relocated, however, because of the narrower track of the MGA.
The brake drums from the TD/TF will fit the MGA rear end.
The second approach is to replace the ring and pinion gears
in the TD/TF with those from the MGA rear end. Although more
elegant, this is not a task for the typical backyard mechanic.
Special equipment is required and the work is close precision.
Moss Motors sells an excellent booklet (part number 212-350),
written by Carl Cederstrand, outlining the ring and pinion
replacement. If you are interested in pursuing this approach,
the booklet is "must" reading.
I recently switched to the MGA 4.3 rear end in my TF.
Although somewhat more expensive, I selected the second option,
changing only the ring and pinion in the interest of
authenticity. This way the original rear end is still basically
intact; externally it is identical to the original setup. (Only
your hair dresser knows!) Because I don't have the specialized
skills or equipment to change the ring and pinion, I shipped my
TF rear end along with the MGA 4.3 ring/pinion gears to O'Connor
Classic Autos in California (phone 408-727-0430). They
specialize in the MGA conversion and for $650 did an excellent
job. As part of the package, they also installed new
differential bearings, pinion bearings and a pinion seal. In
order to reduce shipping costs, I removed the axles, hubs and
brake drums from the rear end that was sent to O'Connor. It was
also an opportunity to inspect the outer bearings and replace the
oil seals at the hubs when reassembling.
I have subsequently learned that Tim Foren in Portland also
performs the MGA 4.3 conversion for less money. Tim may be
reached at 503-254-3876.
Prior to making arrangements with O'Connor, I had to locate
an MGA rear end. Although not plentiful, MGA junk yard parts may
still be found. Following the advertisements in Hemmings Motor
News and after numerous phone calls, I located a MGA rear end at
British Miles in Pennsylvania (1-800-WE FIX MG). If you go with
the first option (completely swapping rear ends), you obviously
will need to purchase a whole MGA rear end. (British Miles
indicates they have more.) If you decide you want to install a
MGA ring/pinion in your TD/TF rear end, you will only need to buy
the "pumpkin" assembly from an MGA rear end, which includes the
ring, pinion and differential gears as a unit. This way you will
get matching gears. The pumpkin that I received from British
Miles was in excellent shape.
In Part II, we will discuss how to adjust the length of the
drive shaft, and how to determine what the rear end ratio in your
car currently is.
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