The external gearbox section covers everything fitted to or directly associated with the outside of the MGB gearbox casing: reconditioned gearbox assemblies, gear levers and knobs, propshafts, speedometer cables and drives, overdrive switches, and the ancillary fittings that connect the transmission to the rest of the drivetrain. Understanding which components apply to your car requires knowing both the gearbox type and, in many cases, whether overdrive was originally fitted.
The MGB used two fundamentally different manual gearboxes across its production life. From 1962 to late 1967, cars were fitted with a three-synchromesh gearbox, which lacked synchromesh on first gear. This gearbox was used with two different engine families: the three-main-bearing 18G and 18GA engines produced from 1962 to 1964, and the five-main-bearing 18GB engine introduced in October 1964.
From the start of the 18GB series, the first motion shaft input spigot was enlarged and the gearbox casing was modified accordingly, which is an important distinction when selecting a reconditioned unit or sourcing associated parts. From late 1967 with the introduction of the Mark II model (series GHN4/GHD4), the fully synchronised four-synchromesh gearbox was adopted and used through to the end of production in 1980. This later gearbox had synchromesh on all forward gears including first, and used a different casing, remote control housing, and rear extension to the three-synchro unit.
Reconditioned gearboxes are available for all variants, including non-overdrive and overdrive versions of both the three-synchro and four-synchro types, covering chrome bumper and rubber bumper cars respectively. Exchange units are supplied on a one-for-one basis in most cases, requiring the customer’s original unit to be returned. Enhanced reconditioning schemes, incorporating a higher proportion of new components over and above standard reconditioning, are also available for customers seeking a higher-specification rebuild.
Gear Lever & Knob History
The gear lever and knob fitted to the MGB changed at several points during production. Early three-synchro cars used a chrome-plated lever with a pear-shaped Bakelite knob. The all-synchro gearbox introduced in late 1967 brought a redesigned, fully vertical lever with a ball-shaped knob.
From August 1972, for the 1973 model year, the knob changed again to a larger pear-shaped design with a clear plastic shift pattern insert, a British Leyland corporate fitment. From September 1976, cars fitted with overdrive used a revised mushroom-shaped knob incorporating the overdrive switch, while non-overdrive cars received a plain cap-style knob. It is important to select the correct gear knob for the specific model year and whether overdrive is fitted, as the fitting method and knob profile differ between applications.
Speedometer Cables & Propshaft
Speedometer cable length and routing also varies across the production run. Three-synchro non-overdrive and automatic gearbox cars used a 48-inch cable, while three-synchro overdrive and four-synchro overdrive cars up to September 1976 used a 60-inch cable. Four-synchro non-overdrive cars required a shorter 39-inch cable, and cars from September 1976 onwards used a 57-inch cable. A right-angle drive adaptor was fitted on all except the four-synchro overdrive gearbox.
Ordering the wrong length is a common error, particularly where the gearbox type has been changed from the car’s original specification.
The propshaft fitted to the MGB is a Hardy Spicer tubular telescopic type with universal joints front and rear. Shaft length depends on the gearbox type and rear axle configuration. Three-synchro non-overdrive cars with the banjo axle originally used a 30-inch shaft, while overdrive versions used a 31 1/8-inch shaft. With the introduction of the tube-type rear axle on the GT from September 1965 and subsequently on roadsters, shaft lengths changed again, and from the introduction of the fully synchronised gearbox with the Mark II model in 1967, the 31 1/8-inch shaft was standardised across all manual gearbox cars through to end of production.
The MGOC Spares range covers external gearbox components including reconditioned gearbox assemblies (standard and enhanced specification), gear levers, gear knobs in both standard and aftermarket styles, propshafts, speedometer cables, overdrive switches and knob cap kits, and all associated fixings.