All MGAs use the standardised BMC B-series rear axle, a three-quarter floating type 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 from the factory, with the nosepiece in natural aluminium. A filler plug sits in the rear differential cover plate and a drain plug at the lowest point of the centre banjo. While the basic axle architecture is common to every MGA variant, several production-change breakpoints during MGA manufacture affect the half-shaft, hub and spline specifications, and the Twin Cam and De Luxe variants use distinct axle arrangements suited to their disc brakes and special wheels.
Axle specification must therefore be confirmed against chassis number and variant before ordering replacement parts.
Axle identification
Each MGA rear axle was individually numbered at the factory. The number, with up to five figures, is stamped on the front of the left-hand rear axle tube. Although these numbers were not entered in the production records, they can be used to confirm that a given axle is the unit originally fitted to a particular car or is a later replacement from another car. The axle number is distinct from the chassis number, the engine number and the gearbox number, each of which is stamped at a different location on the car.
Final drive ratios
The standard final drive ratio on the 1500, 1600 and Twin Cam is 4.3:1 (10/43 crown wheel and pinion); on the 1600 Mk II it is 4.1:1 (10/41). 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. A 3.909:1 (11/43) ratio was available for competition use, and 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 fitted as a production-line item, all required special-order factory specification or post-factory fitment.
The first MGA sales brochure of 1955 also mentioned a 3.727:1 (11/41) ratio, though this is a brochure reference rather than a confirmed production specification. Crown wheel and pinion sets are available as service items matched to each ratio.
Half shafts and hubs, pushrod 1500
The 1500 uses different half shafts, hubs and rear axle casing assemblies depending on whether the car is fitted with disc or wire wheels. From car/chassis 10917 (disc-wheeled cars) and 11450 (wire-wheeled cars), handed rear hub lock nuts were introduced, with a left-hand thread on the left-hand side of the car. Prior to these chassis numbers, right-hand thread lock nuts had been used on both sides of the car. An additional rubber ring oil seal was fitted to the rear hubs of later 1500 models.
Replacement hub lock nuts must match the threading convention for the car's chassis, a left-hand-thread nut will not engage a right-hand-thread hub and vice versa.
Half shafts and hubs, 1600 and 1600 Mk II
From the start of 1600 production, the half shafts, hubs and differential wheels were revised when involute splines were adopted for the half shafts. A further spline revision followed at car/chassis 82893 (disc-wheeled cars) and 82749 (wire-wheeled cars), when the half shafts were given finer splines at the inboard end, changed from 10 to 25 splines. This breakpoint divides the 1600 and 1600 Mk II production into two sub-specifications for half-shaft ordering: pre-breakpoint cars use the coarser 10-spline inboard end; post-breakpoint cars use the finer 25-spline inboard end.
Twin Cam and De Luxe axle
The Twin Cam rear axle was modified to suit the rear disc brakes and the centre-lock Dunlop disc wheels fitted to that model. At Twin Cam car/chassis 1840 the rear hub extensions were modified; at car/chassis 2371 the half shafts were given involute splines, matching the change introduced from the start of 1600 production. The De Luxe variants of the 1600 and 1600 Mk II carried over the Twin Cam rear axle specification, subsequently incorporating the modifications introduced on the later 1600 models as they appeared in production. De Luxe axles therefore differ both from standard pushrod axles (by sharing Twin Cam brake and wheel architecture) and from Twin Cam axles (by including later-1600 spline changes).
Ordering considerations
Rear axle parts selection depends on several overlapping specifications that should be confirmed together. First, variant, pushrod 1500, pushrod 1600 / 1600 Mk II, Twin Cam or De Luxe, which sets the broad axle architecture and brake type. Second, wheel type, disc or wire, which affects hub, half-shaft and casing assembly.