The MGA transmission comprises the clutch assembly, a four-speed manual gearbox with synchromesh on second, third and fourth, a short one-piece propeller shaft, and a rear axle with hypoid bevel final drive. All pushrod MGAs share the standardised BMC B-series transmission architecture, but there are meaningful production-change differences across the 1500, 1600 and 1600 Mk II variants, in gearbox casing, propshaft construction, half-shaft splines and final-drive ratio, that affect which parts suit which car. The Twin Cam uses several components not interchangeable with the pushrod range, and the De Luxe variants sit in a third category, combining pushrod engine and gearbox with Twin Cam-derived clutch and rear axle. Accurate parts selection for any MGA transmission work therefore depends on knowing the car's engine prefix, chassis number and variant identity.
Clutch system
All MGAs use an 8-inch Borg & Beck single dry-plate clutch, type A6-G on pushrod-engined cars, type 8ARG on the Twin Cam. A critical distinction exists in the hydraulic system: pushrod-engined cars (other than the De Luxe) use a combined brake-and-clutch master cylinder with a shared fluid reservoir, while Twin Cam and De Luxe cars have a separate clutch master cylinder. Three main master cylinder patterns were used: the 1500 type, the 1600 and 1600 Mk II type with a raised cover, and the Twin Cam or De Luxe type mounted turned on its side. Identifying which pattern is fitted helps confirm a car's specification when ordering clutch hydraulic components.
Gearbox
The gearbox is the BMC B-series four-speed unit with synchromesh on second, third and fourth gears, first gear is non-synchromesh on all MGAs. The one-piece bellhousing and gearbox casing carries a detachable side cover on the left-hand side. The gearbox was significantly revised at car/chassis 61504, coinciding with the introduction of the 15GD engine, with modifications to the casing, rear extension, third motion shaft and layshaft. Further revisions followed at the start of the 1600 Mk II (chassis 100352) with another casing change, and again at engine number 16GC/4748 when the casing was strengthened with thicker sections and stiffening ribs to reduce vibration. The gearbox rear extension oil seal was a known weak point, with no fewer than five different types used on the MGA 1500 alone, service replacement should be to the correct type for the chassis number fitted.
A close-ratio gearbox was offered as a factory option from engine number 15GB/1379 (March 1959), carrying the engine prefix 16GB-Da when fitted.
Propeller shaft
The propeller shaft is a short one-piece item made by Hardy Spicer, finished in black, with needle-roller-bearing universal joints front and rear. Two distinct patterns were used during MGA production.
The early 1500 specification used a reverse-spline sliding joint between the gearbox output shaft and the front universal joint. From 1500 chassis 20753 a dust cover was added at the front of the propshaft. From chassis 61504 (and on a few earlier cars), the propshaft was revised to incorporate the splined sliding joint within the shaft itself, just behind the front universal joint, and to connect to the gearbox via a four-bolt flange with a matching flange on the gearbox output shaft. This revised propshaft was then carried over without further change to all subsequent 1500 production, to all 1600 and 1600 Mk II cars, and to all Twin Cam cars from the start of Twin Cam production.
Rear axle, banjo type
The rear axle is the standardised BMC B-series three-quarter-floating banjo unit, with hypoid bevel final drive and a one-piece banjo casing with a bolt-on nosepiece and differential carrier. The axle is finished in black with the nosepiece in natural aluminium, and carries a filler plug in the rear differential cover plate and a drain plug on the lowest point of the centre banjo. Axles were individually numbered, with the number stamped on the front of the left-hand axle tube.
Final drive ratios
The standard final-drive ratio on 1500, 1600 and Twin Cam cars is 4.3:1 (10/43 crown wheel and pinion). On the 1600 Mk II this was raised to 4.1:1 (10/41), and the 4.1:1 ratio was also available as a competition-equipment option on earlier cars. A 4.555:1 (9/41) ratio was quoted as an option for 1600 and Mk II models and was also available on earlier cars. For competition use, a 3.909:1 (11/43) ratio was available; Twin Cam cars could additionally be supplied with 4.875:1 (8/39) and 5.125:1 (8/41) competition ratios.
None of these alternative ratios was a production-line item and all required special-order factory specification.