MGB GT Rear Axle
MGB GT Rear Axle
Early MGBs used the “banjo” type differential carried over from the MGA with the rear axle ratio reduced from the MGA’s 4.1 or 4.3 to 3.9 to 1. compensating for the reduction from 15 inch to 14-inch (360 mm) wheels.
MGB GT’s first began using a tube type rear axle in 1967. This unit was substantially stronger being, like the later gearbox, designed for the three litre MGC. All MGB’s used the tube type axle from 1968. These are robust units intended for long life and they will withstand considerable punishment.
The central feature of the tube type differential and axle assembly is the differential carrier to which the ring gear is bolted. The differential carrier also supports the differential and pinion gears which provide the differential action. The pinion gears are held in place by the pinion pin which is inserted through the differential carrier. The entire assembly is lubricated by the differential gear oil. The problem is that each time the car starts and stops, the MAXIMUM cyclic load on the pinion pin occurs at the point where the pin passes through the differential carrier.
After about 80,00 miles of driving, these loading cycles alter the shape of the pinion pin hole from round to oval at the point where the pinion pin EXITS the differential carrier. The oval wearing of the pinion pin hole is the primary cause of the famous MGB rear end clunk which is manifest by the play or slop in the action of the rear axle.
This situation should not be confused with other, far less serious, sources of clunky type noises from the rear of an MGB, including universal joints, suspension, and splined hubs or wheels. Providing that the clunk is not excessive as above, the condition may be minimized by installing thrust washers between the differential and pinion gears and the carrier. However, all MGB tube axles should be checked for the clunk and thrust washers installed between 30,000 and 60,000 miles to reduce the rate at which the problem develops. In time, however, the pinion pin holes in the differential carrier will become worn to the extent that a replacement differential carrier will be needed. This presents a real problem because the differential carrier is not available as a new part! And neither are tube type rear axles!
Top tip! Service your tube type rear axle regularly, like the remainder of your MGB, and install the thrust washers. If you intend to retain ownership of your car indefinitely, you should consider acquiring a ‘good’ spare tube type rear axle for your car. They are quite readily available and relatively cheap now as more MGB’s leave the road to become parts. However, once the current supply is gone, they are gone forever.