The internal gearbox section covers all components contained within the gearbox casing itself: mainshafts, first motion shafts, laygears, layshafts, synchro hubs, baulk rings, bearings, selector forks, thrust washers, gaskets, and overhaul kits. Identifying the correct parts requires a precise understanding of which gearbox type is fitted, and in the case of the three-synchro unit, which generation of laygear is present inside. Getting this wrong is one of the most common mistakes made when sourcing internal gearbox components, and the consequences in terms of part incompatibility are significant. The MGB used two distinct manual gearbox designs across its production life.
The three-synchromesh gearbox was fitted from 1962 to late 1967 and covers cars with 18G, 18GA, and 18GB engine series. The fully synchronised four-synchromesh gearbox was introduced with the Mark II model in late 1967 using 18GD-series engines and remained in use through to the end of production in 1980 on all 18GG and 18V-engine cars. These two gearboxes share no internal components and must be treated as entirely separate assemblies when sourcing parts.
Three-Synchro Laygear Types
Within the three-synchro gearbox, there is a further and critically important division at the laygear. Cars fitted with 18G and 18GA engines, and 18GB engines up to high compression number H74428 and low compression number L58223, were originally equipped with a three-bearing laygear. From the later 18GB series onwards, a four-bearing laygear was fitted. These two designs are not interchangeable, and the associated components, layshaft, thrust washers, bearings, and distance tubes, all differ between the two types.
The original four-bearing laygear has been replaced by a modified design. Owners must identify which laygear is currently installed in their gearbox before ordering any related components, as the distance tube fitted to the original design differs from that required by the replacement. Ordering on the basis of car or engine number alone is not sufficient if the gearbox has been previously rebuilt or had components changed. The three-synchro gearbox also saw a notable development in March 1967 when an improved, larger-diameter layshaft supported on four rather than three bearings was introduced across the range, along with caged needle roller bearings.
This change was phased in by engine number and differs between overdrive and non-overdrive versions, and between high and low compression engines. In December 1965 the second gear synchromesh was also improved, representing a further internal change of relevance when sourcing baulk rings for earlier gearboxes. Within the three-synchro gearbox the first motion shaft is also engine-specific: 18G and 18GA three-main-bearing engines used a specific shaft, while the five-main-bearing 18GB engine required a revised shaft, following the enlargement of the input spigot diameter from 0.62 inch to 0.85 inch when the 18GB was introduced in October 1964. These shafts are not interchangeable.
Four-Synchro Internals
The four-synchro gearbox introduced in late 1967 had a completely new casing and internal arrangement, with synchromesh on all four forward gears. The internal components are common across chrome bumper and rubber bumper four-synchro cars in most respects, though certain thrust washers are application-specific, for example, the front laygear thrust washer is listed specifically for chrome bumper cars. The four-synchro gearbox was used substantially unchanged in its internals through to the end of production in 1980, with very few significant modifications across this period. For those undertaking a full gearbox rebuild on the three-synchro unit, gasket kits are available for both non-overdrive and overdrive variants.
Overhaul kits covering the main wearing components are listed separately for three-bearing laygear gearboxes and four-bearing laygear gearboxes separately. These kits should be identified carefully before ordering. For the four-synchro gearbox, a close-ratio straight-cut gear set is also available, representing the original Special Tuning fitment with revised ratios of 1st 2.45:1, 2nd 1.62:1, and 3rd 1.268:1, the correct choice for high-performance road use or competition. Given the complexity of an internal rebuild in this area, MGOC Spares recommends that owners consider a reconditioned gearbox assembly or five-speed conversion as a practical alternative, particularly where multiple components are required.
The MGOC Spares range encompasses internal gearbox components for both the three-synchro and four-synchro units, including laygears, layshafts, synchro hubs, baulk rings, bearings, thrust washers, selector forks, oil seals, gasket kits, and complete overhaul kits.