The MGB transmission and drivetrain evolved during production, with changes to the gearbox, overdrive unit, and rear axle that directly affect which replacement parts are correct for your car.
Gearbox
From 1962 to 1967, all MGBs used a four-speed manual gearbox with synchromesh on second, third, and top gears, commonly referred to as the 3-synchro gearbox. The introduction of the five main bearing 18GB engine in October 1964 brought changes to the gearbox input shaft, flywheel, and engine backplate, meaning the transmission for three main bearing cars (1962 to 1964) is not directly interchangeable with its five main bearing successor (1965 to 1967) without changing these associated components. With the MkII in late 1967, the gearbox was replaced by a fully synchronised four-speed unit, the 4-synchro, originally developed for the MGC and considerably over-engineered for the MGB, a characteristic owners have always welcomed. The 4-synchro gearbox continued to the end of production in 1980, though the casing, first gear ratio, and speedometer drive were revised for rubber bumper models from September 1974.
The Borg-Warner Type 35 automatic gearbox was offered from late 1967 and discontinued in 1973. MGB automatics are identified by engine prefix codes containing ’Rc’.
Overdrive
Overdrive was available as a factory option from early 1963, becoming standard on UK cars from 1976. The original D-Type Laycock de Normanville unit was revised from 1968 to the LH-Type.
Chrome bumper cars from 1968 to 1974 used a unit identifiable by its black specification label, while rubber bumper cars from 1975 used a blue label unit, the essential difference being the speedometer pinion gear tooth count. The overdrive switch position changed across production, from a dashboard toggle, to the distinctive cranked toggle known as the Shepherd’s Crook from April 1965, to steering column stalk activation on rubber bumper cars, and finally to a gearknob-mounted slider from September 1976.
For owners of non-overdrive cars weighing the cost of an overdrive conversion against an aftermarket five-speed gearbox, the comparison is worth exploring, a five-speed can represent better long-term value, particularly where the cost of a reconditioned overdrive gearbox and unit is set against the practical benefits of modern construction and well-matched ratios.
Clutch
All manual MGBs used a single dry plate 8-inch spring clutch, hydraulically operated, throughout production. When replacing the clutch, always treat it as a three-piece job, friction plate, pressure plate, and release bearing replaced together as a kit. The clutch fork pivot pin and bush should be inspected and replaced as a matter of course, and a clutch alignment tool used to centre the friction plate during assembly.
Rear Axle
Early MGBs from 1962 used the Banjo-type rear axle carried over from the MGA, with a removable aluminium differential accessed from the front of the axle. Wire wheel hub thread pitch changed from 12 TPI to 8 TPI in 1964.
From July 1967, the Salisbury tube-type axle was phased in, heavier but quieter and more robust. The tube axle went through two variants: pre-September 1976 without rear anti-roll bar mounting points, and post-September 1976 with dedicated mounting points and a revised handbrake compensator arrangement. A genuine factory wire wheel axle measures 46 inches across the casing versus 48 inches for a steel wheel axle, a two-inch difference that affects hub and wheel fitment and is the clearest way to identify which type is fitted.
Propshaft
Three propshaft lengths were used, 30 inches, 31? inches, and 32 inches, determined by gearbox type, overdrive fitment, and axle type. Vibration at speed typically indicates worn universal joints rather than a failed propshaft. The MGOC Spares range includes reconditioned gearboxes, overdrive units, five-speed conversion kits, clutch kits, rear axle components, differentials, propshafts, and all associated transmission hardware, supported by expert technical advice for every MGB drivetrain configuration.