The three-synchromesh gearbox was fitted to the MGB from the start of production in 1962 through to the introduction of the all-synchromesh four-speed unit in late 1967. It covers cars with 18G, 18GA, and 18GB engine series and has synchromesh on second, third, and fourth gears only, with first gear unsynchronised. While the basic architecture of the gearbox remained consistent across this period, several significant internal changes occurred that directly affect which parts are required for a rebuild.
Laygear Identification
The most critical distinction within the three-synchro gearbox is the laygear type. Cars fitted with 18G and 18GA engines, and 18GB engines up to high compression number H74428 and low compression number L58223, originally left the factory with a three-bearing laygear.
Later 18GB engines were fitted with an improved four-bearing laygear as the layshaft design was progressively developed, culminating in the introduction of a larger diameter layshaft on four rather than three bearings in March 1967, along with caged needle roller bearings. These two laygear generations share no components, the layshaft, thrust washers, bearings, and distance tubes are all specific to each design and are not interchangeable. Before ordering any laygear-related parts, it is essential to identify which type is physically present in the gearbox, as the car's engine number alone may not be sufficient if the gearbox has been previously rebuilt.
The original four-bearing laygear has been replaced by a modified design, which requires a different distance tube to the original unit. The three-bearing laygear and its uprated layshaft remain available.
First Motion Shaft & Other Changes
The first motion shaft also differs depending on the engine. Three-main-bearing engines (18G and 18GA) use a specific shaft, while the five-main-bearing 18GB, introduced in October 1964, requires a different shaft following the enlargement of the input spigot from 0.62 inch to 0.85 inch. These are not interchangeable, and the shaft must match the engine fitted in the car.
The second gear synchromesh was improved in December 1965 as a production change, and the reverse selector and detent were modified in September 1966 at the same time as a reverse light switch was added. Where baulk rings are being replaced on earlier gearboxes, it is worth noting that both steel and bronze second gear baulk ring types have been available historically, these are listed as alternatives for that position. The MGOC Spares range covers internal components for the three-synchromesh gearbox including the three-bearing laygear assembly, four-bearing laygear components (caged needle roller bearings, distance tube, thrust washers), uprated layshaft, 3rd motion shaft bearings, 1st motion shaft bearing, needle rollers, synchro hub, baulk rings (3rd/4th gear and 2nd gear types), detent balls and springs, selector forks (1st/2nd, 3rd/4th, and reverse), tailshaft oil seal, first motion shaft shims, Woodruff key, and gasket kits for both non-overdrive and overdrive applications.