The internal components of the MGA gearbox comprise the first motion shaft (the input shaft from the clutch), the third motion shaft (the mainshaft carrying the output gears), the layshaft (running on parallel bearings below the third motion shaft), the synchromesh hubs and rings fitted to second, third and fourth gears, the selector forks and rails, and the bearings and thrust washers that support the rotating parts within the casing. The MGA gearbox is the standardised BMC B-series four-speed unit with synchromesh on second, third and fourth gears; first gear and reverse are non-synchromesh on all MGAs. Several significant internal specification changes were made during MGA production, and correct parts selection for an overhaul or component replacement depends on identifying the specific gearbox fitted to the car.
Gear ratios
The standard MGA gear ratios are: First 3.64:1, Second 2.214:1, Third 1.374:1, Fourth 1.0:1, Reverse 4.76:1. These gearbox ratios are common across all MGA variants, 1500, 1600, 1600 Mk II and Twin Cam. The overall gear ratios differ between variants only because the final drive ratio is different: the 1500, 1600 and Twin Cam use a 4.3:1 final drive, giving an overall fourth-gear ratio of 4.3:1 at the road wheels; the 1600 Mk II uses a 4.1:1 final drive, giving an overall fourth of 4.1:1.
Replacement gears for a standard gearbox therefore suit every MGA variant equally, with the overall ratio set by the final drive in the rear axle rather than by the gearbox itself.
Production-change breakpoints
The gearbox was significantly revised at car/chassis 61504, coinciding with the introduction of the 15GD engine. At that breakpoint the casing, rear extension, third motion shaft and layshaft were all modified. A further change followed at car/chassis 100352 (start of 1600 Mk II production) when the casing assembly was modified again. At engine number 16GC/3929 the gearbox first motion shaft was given finer splines, with a correspondingly modified clutch driven plate introduced at the same breakpoint. At engine number 16GC/4748 the casing was strengthened with thicker sections around the bellhousing and stiffening ribs, introduced to reduce vibration.
Each of these breakpoints affects which internal parts are correct for a gearbox of known chassis or engine number, parts from different breakpoints are not necessarily interchangeable and the gearbox specification should be confirmed before ordering.
Close-ratio gearbox option
A close-ratio gearbox was available as an option from engine number 15GB/1379 (March 1959). When the close-ratio gearbox is fitted, the engine prefix is changed to 16GB-Da as a marker of the different installation. The close-ratio ratios are: First 2.445:1, Second 1.62:1, with Third and Fourth continuing closer to the standard set. The close-ratio gearbox has detail differences from the Twin Cam gearbox but its internals are, for practical purposes, identical to the Twin Cam specification.
Close-ratio gearboxes were factory-fitted to competition-specification cars and were available retrospectively as an option on other models.
Synchromesh
Synchromesh is fitted to second, third and fourth gears on all MGA gearboxes. First gear is non-synchromesh, as is reverse. Worn synchromesh is one of the most common reasons for a gearbox rebuild on cars that have seen extended service, the synchro hubs and rings progressively lose their ability to equalise input and output speeds during a shift, and the symptoms are difficulty engaging gears cleanly and grinding noises during the change.
Replacement synchro hubs, rings and their associated springs and detents are available as service items and are typically renewed as a set during a gearbox overhaul rather than individually.
Gearbox identification
Each gearbox carries an individual number stamped into the casing adjacent to the dipstick location on the right-hand side. Gearbox numbers are four-figure numbers with varying letter prefixes, though the precise factory numbering system has never been fully decoded. The gearbox number is the starting point for confirming which internal specification applies to a given unit, particularly where a car has had its original gearbox replaced during its service history and the chassis number alone no longer identifies the gearbox fitted.
Ordering considerations
Internal gearbox components must be matched to the specific gearbox variant. The gearbox number stamped adjacent to the dipstick is the primary identifier; the chassis number and engine prefix help confirm which breakpoint specification (pre- or post-61504, pre- or post-100352, pre- or post-16GC/3929, pre- or post-16GC/4748, and standard versus close-ratio) applies.