The MGB was produced with two distinct manual gearbox types across its production life, plus an optional automatic transmission. Understanding which gearbox your car is fitted with is essential before ordering any internal or external components.
Manual Gearbox Types
From 1962 to 1967, the MGB used a 3-synchromesh gearbox without synchronisation on first gear. This unit was paired with both 3-main-bearing (18G/GA) and 5-main-bearing (18GB) engines, each requiring a different reconditioned assembly. The 3-synchro gearbox was available with the Laycock D-type overdrive, operating on third and fourth gears.
From 1968 onwards, the 4-synchromesh gearbox was introduced, providing synchromesh on all four forward gears and remaining in production through to 1980. The 4-synchro gearbox used the Laycock LH-type overdrive, also operating on third and fourth gears but sharing no parts with the earlier D-type unit. Overdrive cars from 1968 to 1974 use an LH-type unit identifiable by a black Laycock plate (1280 TPM), while rubber bumper cars from 1975 onwards use a blue plate unit (1000 TPM), this distinction matters when selecting a reconditioned unit. A Borg-Warner Type 35 automatic gearbox was offered as an option from 1967 through to August 1973.
Internal components for this unit are listed separately as many parts are no longer readily available; a reconditioned assembly is generally the most practical approach.
Five-Speed Conversion
For owners looking to modernise their MGB's gearing, a 5-speed conversion based on the Ford Sierra Type 9 gearbox is available as a complete bolt-on kit. This retains the same effective top gear ratio as the standard overdrive fourth gear, while adding synchromesh on all five gears and improved acceleration through a higher second gear ratio. Kits are available for all MGB variants from 1962 onwards, with early 3-synchro cars (pre and post-1965, banjo and tube axle), 4-synchro chrome bumper cars (1969 to 1975), and rubber bumper cars (1976 onwards) each requiring a specific kit. An important note on early 3-synchro gearboxes: 18G and 18GA engines, and 18GB engines up to certain build numbers, were fitted with a 3-bearing laygear which differs significantly from the later 4-bearing type.
This affects internal component selection and should be confirmed before ordering any internal 3-synchro gearbox parts.