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MG MGA - transmission swap

hi there
I have early MGB 3 synchro+overdrive tranny with bad second gear in my MGA. I also have MGB 3 synchro no overdrive on bench.
How easy or how difficult, labor+financial, it will be to move overdrive unit from present bad gear tranny to rebuilt one. Will that be a simple swap of rear housing+overdrive and possibly main shaft or will it involve major surgery and large bank loan?
There is also option of fixing the second gear and what ever else is amiss. No I am not interested to go to 5 speed or 4synchro, I am happy with the overdrive.
All and any suggestions/opinions are welcome.


To swap the OD unit to a standard gearbox requires transferring the main shaft. This would require major disassembly of two units and reassembly of one. Repairing (rebuilding) the unit with OD requires one disassembly plus reassembly, which should be cheaper (less work) when you don't have to tear into the second unit. The only reason to disassemble the second unit may be to scavenge some good parts to rebuild the first one.
Barney Gaylord

A question, Barney. I have an early 1500 g/b (low starter) in very good condition. I have a 1600 case (high starter). Will the internals from the 1500 work in the 1600 case? I do have a spare 1600 to rob parts from. Thanks, Barney, I appreciate your efforts. Marv Suart
Marvin Stuart


The internals are the same apart from the back of the output shaft, which is either flanged or splined according to the type of gearbox. All the levers, rods, gears and bearings are the same in 1500 and 1600 boxes, whether pushrod or Twin Cam. A lot of parts in the early 1800 boxes are also the same, just the laygear& shaft has a larger diameter
dominic clancy

Dominic, the MGB laygear was identical to the MGA item until the last year (1967) when they changed the diameter. In fact you don't see a large shaft box all that often.

The splines on the input shafte vary as well with late Mk 2 MGA and all MGB being fine spline with earlier ones being coarse.

But as you said, a huge amount of interchangeability.
Bill Spohn

Marvin, -- The short answer is yes, but watch the input and output shafts.

For MGA gearboxes only, there are three different main shafts (output shafts). Early 1500 has 1-inch 10-splines, mid 1500 has 1-1/16" 10-spines, late 1500 (January 1959) and all later have flanged output. There are also three different tail housings, and you must use the tail housing that matches the output shaft. There are also three different prop shafts that must also match the gearbox output.

There are two different input shafts. Most MGA have 10-spline. Late production MGA 1600-MK-II has 23 splines. This is an easy fix, just use the clutch disc that matches the splines.

For all MGA and MGB through 1967, everything between the front and center main ball bearings is interchangeable. The sole exception is late 1967 production has larger layshaft. That larger layshaft and matching laygear and needle bearings can be retrofit to earlier units by reaming out the mounting holes in the main case.

Late MGA and Early MGB tail housings are interchangeable, so you can install a late MGA tail housing on an early MGB gearbox to install it in the MGA (but the speedometer drive gears are different).
Barney Gaylord

Thanks everyone,I appreciate all the excellant help. Marvin
Marvin Stuart

hi marvin
Why do you not start your own thread, ?!?!?!? do not bother to say sorry my mind is made up about you


This thread was discussed between 24/10/2011 and 25/10/2011

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