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MG Car Repair/MGB transmission swap


Hi Howard, 1968 MGB with a tired 4 speed transmission.  I have a lead on a 4 speed with an overdrive unit probably from a 70s car.  Would this be a feasible swap, or would I be better off rebuilding what I already have?  I do recognize that the overdrive transmission may need rebuilding as well...

Thank you in advance for any and all information!


Hi Dan,

look at your starter solenoid on your car. If it is mounted on top of the starter motor then the overdrive transmission is a bolt in replacement for the 4 speed. If however your starter solenoid is a remote mounted solenoid, then it is not a bolt in unless you can get the engine backing plate and starter motor from the 70's car. Your car should have the solenoid on the starter motor and just be a bolt in.

However, in the 60's dealerships sometimes still titled cars the year they sold them not the year the car was suppose to be. Those earlier MGB's had a different bell housing and engine backing plate and different starter motor and solenoid thus it required some work to put a later model OD trans in one.

The overdrive transmissions are highly prised as they make the MGB able to cruise on the US expressways. It is even more of an advantage on doing a swap because your 4 speed rear end is geared slightly higher then an OD model which makes it geared even higher then an OD model. The engine is strong enough to use this higher gearing on highways but at times on long steep grades you should drop it out of overdrive so you don't "Lug" the engine.

The standard 4 speed box is great for winding country roads, but the overdrive is needed for our expressways. You will need to add wiring and a switch to operate it. The overdrive is a two speed automatic transmission on the back of a standard 4 speed box. An electric switch turns it on through a 3rd / 4th gear switch on the box. This is so you don't engage the over drive in 1st or 2nd gear. Also it prevents engagement in reverse. If this system is bypassed and reverse is engaged, it will for sure explode the over drive unit due to a Sprag bearing in the overdrive, so DO wire in the switch on the transmission.


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Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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