Rear Axle Lubrication, Hub Oil Leaks and Hub Replacement

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Written by B S Fine

Like the gearbox, the rear axle only requires regular attention to its lubrication until signs of wear dictate that more significant maintenance is needed. The service intervals are the same: check and top up every 1,000 miles (1,600kms) and drain/replace every 6,000 miles (9,600kms). In this case it is essential to use the right specification of Hypoid oil. The oil is distributed during normal use to the extremities of the axle casing so that the hub bearings are lubricated internally too.

Filling is via a plug in the rear face of the differential casing. The plug also establishes the filling level. One simply adds oil until it overflows from the hole, so don’t lie right underneath it! A gun or plastic container with a flexible nozzle is the best way to top up. It is a good idea to check that the breather is also clear because pressure build up in the casing will expel oil from any weak points in the joints.

The most recurrent problem with the rear axle is a tendency to leak at the hubs. This is often caused by the pressure described above. Unfortunately, once the oil finds its way out, the spinning hub distributes it evenly over the brake drum and braking can become dangerously unbalanced. Any sign of contamination is an immediate MOT failure.

The rear part of the hub is fixed on the end of the axle and held in place by a large octagonal nut. This is nothing to do with brand loyalty: the nut has limited space inside the hub and eight flats can be accommodated more easily in the diameter than six without sacrificing the amount of metal in the nut. A special box spanner is needed if you have to withdraw the hub. It is obtainable from MG specialists but is expensive, so borrow one if you can.

The outer face of the hub is actually a flange on the end of the half shaft and transfers the drive to the hub. In order to remove the differential, the half shaft must be partially withdrawn to extract its inner splined end from the planet gears in the diff.

If you detect signs of oil leakage, it will be necessary to replace the oil seals in the axle.  Early axles had two seals per side: a sprung rubber seal at the rear of the bearing and a paper gasket between the outer and inner parts of the hub. Later axles also have a rubber sealing ring behind the paper gasket, set into a groove machined into the joint face on the hub. As leakage is a common problem here it is a good idea to use a smear of non-setting jointing compound on both side of the paper gasket. Last time I fitted a new rubber sealing ring, it proved to be slightly smaller than its groove, so it was almost impossible to make it sit in its groove while tightening up the half-shaft flange. In the end, I found that by stretching the ring before fitting, it would hold a slightly expanded size long enough to quickly close the joint.

 While the rear wheels are off and the axle parts dismantled this far, it is only a few steps further to replace the bearing. This involves undoing the octagonal nuts, pulling the hub off the axle, driving the old bearing out and the new one in. From car number 12639, the left hand hub nut has a left hand thread but take care because your axle may have been changed by a previous owner. The hub nuts are held by a tab washer that needs tapping away before you apply the spanner.

Since the bearings are not adjustable, replacement is the only way of dealing with wear and there is no harm in doing the work proactively before your wheels show undue play. It is one of those “I may as well do it while I’m down here” jobs and repays the additional cost with miles more trouble-free motoring. No grease packing is required but it is advisable to smear the new bearing with hypoid oil so that it is protected during the brief period until internal lubricant reaches it.