The Midget rear axle is a semi-floating live axle with a hypoid bevel differential, common in layout to both the 1275cc and 1500cc cars. The axle case, half shafts, and hub assemblies differ between steel wheel and wire wheel applications, and the differential ratio determines the car's overall gearing, selecting the correct combination for the car's wheel type and intended use is essential when ordering replacement components.
Final Drive Ratios
Two final drive ratios were used: 3.9:1 (standard) and 3.7:1 (alternative). The 3.9:1 ratio provides livelier acceleration and is suited to normal road driving, while the 3.7:1 offers more relaxed cruising at higher speeds with reduced engine revs. Complete reconditioned differentials are available on an exchange basis, either with the existing crown wheel and pinion or with a new set fitted. The differential cage, sun wheels, planet gears, and cross pin are not separately available as new components, making the exchange reconditioned unit the most practical option for a worn differential.
Differential Rebuild
The differential is secured to the axle case by eight studs with a gasket between the flanges. The pinion is carried on taper roller bearings with a collapsible spacer controlling the preload. Setting the pinion preload and crown wheel backlash correctly requires specialist knowledge and equipment, an incorrectly preloaded pinion bearing will fail rapidly, and incorrect backlash causes whining and accelerated tooth wear. Differential rebuilds are best left to an experienced specialist or supplied as a reconditioned exchange unit.
Half Shafts
Half shafts are specific to the wheel type. Steel wheel half shafts have a pressed-on hub with four wheel studs. Wire wheel half shafts have a flanged end that bolts to a splined hub adaptor carrying the knock-on spinner. A competition-grade wire wheel half shaft is available for motorsport use, providing additional strength for high-power or high-grip applications.
The rear wheel bearing kit and oil seal should be renewed whenever a half shaft is removed, as disturbing the bearing is likely to compromise its seating. The hub nut thread direction differs between left and right sides of the car (left-hand thread on the left, right-hand thread on the right) to prevent the nuts working loose under driving and braking loads.
Axle Case Variants
The axle case itself differs between steel wheel and wire wheel cars, and between the 1275cc and 1500 applications. The axle case specification was revised during 1500 production, so the chassis number should be confirmed before ordering a replacement axle case to ensure the correct version is supplied. A breather on the top of the axle case prevents pressure build-up that would otherwise force oil past the half shaft seals during hot running. The filler and drain plugs are common square-drive items.
Axle Oil
The axle oil specification is EP80/90 gear oil, and the level should be checked periodically as part of routine maintenance. Topping up is achieved through the side-mounted filler plug, the correct level is when oil is just at the bottom of the filler hole. EP-rated gear oil is essential for the hypoid gear set in the differential, as the high contact pressures generated by hypoid teeth require the extreme pressure additives present in EP oils.