Overdrive was one of the most desirable factory options on the MGB throughout production, and remains highly sought after today for the relaxed cruising and improved fuel economy it provides. Two fundamentally different overdrive units were fitted across the production life of the MGB, and it is essential to identify which type is on your car before ordering any components. The two units share no parts and are not interchangeable.
D-Type Overdrive (1963 to 1967)
earlier unit is the Laycock de Normanville D-type overdrive, fitted to the three-synchromesh gearbox from early 1963 through to the end of three-synchro production in late 1967. This covers cars with 18G, 18GA, and 18GB engine series. The D-type operates on third and top gears and has a reduction ratio of 0.802:1, giving an overall overdrive top gear ratio of 3.135:1 against the standard MGB final drive of 3.909:1. On the three-synchro gearbox, cars originally fitted with overdrive can be identified by the letter R in the engine number prefix, for example, 18GRU-H rather than 18G-U-H.
The overdrive switch on these cars was originally mounted at the end of the facia on the driver’s side, and from April 1965 took the distinctive ‘shepherd’s crook’ switch shape.
LH-Type Overdrive (1967 to 1980)
The later unit is the Laycock LH-type overdrive, introduced with the all-synchromesh four-speed gearbox from the Mark II model in late 1967. The LH-type has a reduction ratio of 0.82:1, giving an overall overdrive top gear ratio of 3.205:1. On four-synchro cars, the overdrive operates on third and top gears. The LH-type overdrive can be identified by its identification plate colour: chrome bumper cars from 1968 to 1974 carry a black Laycock label, while rubber bumper cars from 1975 onwards have a blue label.
This distinction matters because the two variants differ internally in their speedometer drive gearing, the chrome bumper unit uses an 8-tooth drive gear, while the rubber bumper unit uses a 6-tooth gear, and the reconditioned units are listed separately to reflect this. The MGB GT V8 was also fitted with the LH-type overdrive as standard equipment, though on early V8 production cars it operated on third and top, later being revised to top gear only. Overdrive became increasingly standard across the MGB range during the rubber bumper years.
From June 1975 it was standard equipment on home market four-cylinder cars, and from June 1976 it was standard on all right-hand drive cars. From the start of the 1977 model year, the overdrive switch was integrated into the gear lever knob on cars so equipped, replacing the earlier stalk-mounted switch introduced on rubber bumper cars at the start of 1974 production. There is also an important detail regarding the early LH-type gearbox mainshaft. The very earliest four-synchro overdrive gearboxes used a Woodruff key to drive the oil pump cam on the mainshaft, whereas later units used a circlip-located arrangement. This affects which mainshaft-related components are required when working on the overdrive end of an early four-synchro gearbox.
The MGOC Spares range includes reconditioned D-type overdrive units on a one-for-one exchange basis, and reconditioned LH-type units in both chrome bumper and rubber bumper specifications, both on an exchange basis. Supporting components available across both sections include solenoids, gaskets, oil pump assemblies, seals, O-rings, bearings, annulus components, speedo drive pinions and housings, studs, circlips, cam parts, and a reconditioned sliding member for the D-type unit.