The four-synchromesh gearbox was introduced on the MGB with the Mark II model in late 1967, fitted to all cars from the 18GD engine series onwards through to the end of production in 1980. It was used across the full range of four-cylinder MGB models, both roadster and GT, and with the MGB GT V8, which used the same basic gearbox design with a different casing to accommodate the larger clutch. The four-synchro unit was also common to the MGC and the Austin 3-litre saloon. It has synchromesh on all four forward gears including first, which was the principal improvement over the earlier three-synchromesh gearbox.
Internal Components
The internal architecture of the four-synchro gearbox remained essentially unchanged from 1967 to 1980, with very few significant modifications recorded across the production run. One detail of relevance for overdrive-equipped gearboxes concerns the oil pump cam drive on the mainshaft: very early four-synchro overdrive gearboxes used a Woodruff key to drive the cam, while later production used a ball-located cam retained by a circlip. Both the late-specification ball-located cam and the circlip are available, and the Woodruff key for the early mainshaft specification is also available. Identifying which mainshaft type is fitted is necessary before ordering these components.
The baulk ring used in this gearbox is a single part number covering all four synchromesh positions, first, second, third, and fourth, as the same ring is used throughout. This is a wear item that should be inspected during any gearbox rebuild. The laybearing and laygear bearing retaining circlip are listed for the four-synchro unit’s layshaft, and the shared 3rd motion shaft front bearing serves both the three-synchro and four-synchro gearboxes in this position. The non-overdrive 3rd motion shaft centre bearing and the Woodruff key for the non-overdrive speedometer drive gear application are also available.
The range of internal components covered for the four-synchro gearbox is narrower than for the three-synchro unit. Owners undertaking a full internal rebuild of the four-synchro gearbox are advised to consider a reconditioned gearbox assembly or a five-speed conversion as a more practical alternative where multiple internal components are required. The MGOC Spares range includes internal components for the four-synchromesh gearbox including the 3rd motion shaft centre bearing (non-overdrive), 1st motion shaft bearing, needle roller bearing, baulk ring (all positions), detent balls and springs, laybearing, laygear bearing retaining circlip, tailshaft oil seal, propshaft bolt, ball bearing, LH-type overdrive oil pump cam, overdrive cam circlip, first motion shaft locktab, shim, Woodruff key, and gasket kit for four-synchro overdrive applications. Customers with requirements for components not listed here are encouraged to contact the MGOC Spares team for sourcing guidance.