The rear axle and propshaft section covers the final drive assembly and the connection between the gearbox and the driven wheels. The rear axle is the component most significantly affected by the BSF-to-Unified thread change at chassis TD/12285, and the crown wheel and pinion ratio determines the car's overall gearing and cruising character.
The BSF to Unified Thread Change
At chassis TD/12285 in late 1951, the entire rear axle changed from BSF to Unified (UNF) thread. This is one of the most comprehensive single production changes in the TD specification, affecting cover studs, plugs, differential cage, pinion pin locking bolt, crown wheel bolts, half shaft nuts, bearing housing bolts, pinion flange nut, and wheel studs and nuts. Every fastener must match, Unified and BSF cannot be mixed within a joint. The chassis number is the definitive reference for thread type.
Crown Wheel and Pinion Ratios
Complete crown wheel and pinion sets are catalogued in three ratios, each in both BSF (TD to chassis 12284) and Unified (TD from 12285, all TFs) thread. The 5.125:1 is the standard TD ratio for strong acceleration. The 4.875:1 was optional on the TD and became standard on Mark II and TF, suiting the Mark II's higher-compression specification. The 5.444:1 provides the shortest gearing for low-speed pulling or competition.
A fourth ratio, 4.55:1, was a factory TD option providing the lowest cruising rpm, not currently available as a complete set, though speedometer drive components for cars originally fitted with it are available. A crown wheel and pinion must always be replaced together as a matched set, shimmed correctly for gear mesh pattern and bearing preload. The matched bolts for the crown wheel also differ between BSF and Unified thread specifications, and lock tabs retain the bolts once fitted.
Differential and Internal Components
The differential sits within the axle nosepiece. The cage, pinion pin locking bolt, and differential and pinion gears are thread-specific, BSF and Unified differ, reflecting the 12285 change. The pinion locating pin, differential bearings, collars, and thrust washers are common. Bearing collars come in twelve thicknesses from 0.495 to 0.506 inch (0.001-inch increments) for precise shimming.
The pinion bearing set, spacer, and seal are universal; pinion distance washers come in twelve thicknesses from 0.117 to 0.128 inch. Correct shimming is critical to quiet operation and long life, specialist work.
Propshaft
The Hardy-Spicer propshaft connects gearbox to rear axle through flanged joints, available new and on a customer's own unit reconditioned basis. The two universal joints come in greasable (periodic lubrication) or sealed (maintenance-free) types. A grease nipple also serves the front sliding spline; an improved seal and cap is available to address weeping. The eight flange bolts and nuts are common across all specifications.
Half Shafts, Rear Hubs and Brake Drums
Half shafts transmit drive from differential to rear wheels. Replacement shafts are supplied in Unified thread and can replace BSF originals when fitted with matching Unified nut and washer. Rear hub bearings, oil seals, and seal collars are common across all specifications. The rear hub and drum arrangement differs by wheel type: steel-wheeled cars use integral drums with five pressed-in studs (BSF to chassis 12284, Unified from 12285); wire-wheeled cars use splined hubs with a drum on twelve studs, nuts, and lock tabs.
Wire Wheel Hubs and Spinners
Wire-wheeled cars use centre-lock hubs and spinners in place of the steel-wheel bolt-on arrangement. Spinners are available in two styles, two-ear and octagonal, each in left- and right-hand threads to suit the appropriate side of the car, and all carry the MG logo. A complete wire wheel conversion kit is available for cars originally supplied with steel wheels, for both TD and TF models, containing the splined hubs, brake drums with their fixings, two-eared spinners, a hammer, and the spare wheel carrier adaptor and spare wheel spinner.
Ordering Considerations
The chassis number is the single most important identifier when ordering rear axle components, it determines whether BSF or Unified thread components are required. Wheel type (steel versus wire) determines the rear hub and brake drum specification. The axle ratio is needed only when ordering crown wheel and pinion sets or matching speedometer drive components. The axle cover gasket should be ordered alongside any work that involves removing the rear cover to access the differential, as it should always be renewed rather than reused.