>If you have a 73 MGB GT with a four speed box, can you "bolt on
>the overdrive or must your replace the whole box with one with an overdrive.
I looked into such a conversion on the '74 MGB that I had several
years ago and it wasn't economically feasible at that time, but I
learned a lot about what's required and I've been gradually collecting
pieces since then to convert my '71 one day (unless I find a nice
Magnette first...)
To fit the O/D unit on the back of the MGB transmission, you have to
change not only the transmission housing but also the tailshaft itself.
This means getting into the guts of the transmission, since the tailshaft
is the last thing that comes out when disassembling. It's probably a lot
easier just to find a complete gearbox (such as the one that I have sitting
in the front of my garage, neener neener neener) with O/D and install it.
Then there's the issue of driveshaft length. The O/D unit makes the
transmission slightly longer than the non-O/D box; to compensate, the
O/D cars were equipped with shorter driveshafts. I don't know the
difference off the top of my head, but this is actually easy to fix.
Find a good driveshaft shop -- there's a national chain called Driveline
Services that I've had do work on the race car -- and have them shorten
the driveshaft to fit your needs. What they prefer is for you to
measure the distance between the flange at the back of the transmission
and the flange at the front of the diff, then they will calculate the
correct length for the driveshaft (which needs to allow for slip in
both directions). You can also see how much you need to remove and
have them cut that off, taking the 1.5" of yoke slippage into account.
That's what Danny and I did on the black car. Whatever you do, the shop
I went to welded and balanced the driveshaft, and also gave it back with
a shiny coat of black rustproofing paint on it. (I also had them install
a new U-joint, since one of them was almost completely shot.)
The last thing to consider is wiring. There are three different switches
to control the O/D. The early cars had a switch on the dashboard; later
the switch was moved to the wiper stalk, and on the newest cars it was
placed under a cap at the top of the shift knob, like the ejector seat
button in James Bond's DB5. It's probably easiest to install a new
switch and mount it somewhere convenient, but it's coolest to follow the
factory method for your year of MGB; that should be the wiper-stalk switch,
but I'd have to look it up. So add a switch, and possibly some wiring
harness pieces, to your shopping list. Remember that the stock O/D is
set up not to engage in 1st, 2nd or reverse, so you'll need some kind
of sensor to tell the box what gear you're in and feed this information
back to the solenoids that engage the O/D clutches.
It's not a trivial task. It's not as complicated as, say, swapping in
an engine from another vehicle, but there are a lot of little changes
that can make the job less than straightforward if you start working
on it before you know everything you need to have. I'm still not sure
I've got it all, but I'm content to wait.
--Scott
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