MGB Differential Rebuild

MGB Differential Rebuild

This guide captures ‘bygone’ advice on how to perform an MGB Differential Rebuild.

This post gives an Introduction. The full series is as follows:

To see all my ‘bygone’ advice guides please visit Classic Car Maintenance DIY Guides.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.





Differential Noise

The differential is a component that is unlikely to give any trouble unless regular lubrication has been omitted from servicing schedules, as it is normally a hard­wearing and long-lasting unit.

However if any un­usual noises develop at the rear of the car it is possible that the differential is at fault and it should be checked and serviced immediately.

Do not delay if you suspect that the unit may be worn or damaged, continued use will accelerate any harm already done and the com­plete differential unit may have to be renewed as a result.


Rear Axle Noise

A knocking noise emanating from the rear of the car under acceleration or a continuous whine at all speeds can indicate that the differential is at fault.

However first check that these symptoms are not caused by a worn universal joint or by faulty wheel bearings.


Semi Floating Rear Axles

Semi-floating rear axles incorporate a bearing on the inside of each end of the axle casing and are a popular classic car design. They were used on most classic cars with a live axle.

This post describes the removal and overhaul of the semi-floating rear axle which is fitted to the BL MGB manufactured from 1968 onwards.

Earlier models of this car were equipped with three-quarter­ floating rear axles which can be serviced without removing the axle casing from the car. I will be writing about the in time.


MGB Rear Axle Exploded View

MGB Rear Axle Exploded View
This exploded view of an MGB “Salisbury” semi-floating rear axle shows the components which are involved in the overhaul

Preparations for an MGB Differential Rebuild

The overhaul of this type of semi-floating axle involves difficult and time-consuming work and it is not a job for the inexperienced DIY mechanic.

A great deal of precise work and a number of fairly complicated calculations are necessary, since there are many critical specifications which allow no margin of error.

A number of special tools are also essential (see below) and the work should not be attempted if these tools are not at your disposal.

It is therefore recom­mended that you read carefully through the whole all aspect / relating posts before deciding whether or not the overhaul is within your capabilities.

However if you decide that you cannot carry out the work, you may still find that money can be saved by removing the rear axle assembly from the car yourself and then taking it to a specialist for overhaul.

If your car’s differential unit does display symptoms of wear it should be overhauled promptly since continued use will increase the stress on fatigued components and can lead to complete rear axle failure.


Tools For The Job

It is important to note that this overhaul requires the use of several special tools in addition to the normal range of hand tools which the DIY mechanic is likely to possess.

  • A torque wrench which will register readings of up to 20.75 kg/m (1501b/ft) is absolutely essential for the job.
  • A dummy pinion shaft is needed to ensure correct location of the actual pinion and in order to set the crown wheel precisely, a special jig was once supplied by BL in three components: 18G 191, 18G 191F and 18G 191 J.
  • In order to extract the axle shafts from their housings BL supplied an impulse extractor, 18G 284 and adaptor 18G 284 D, but if this is not available an ordinary slide-hammer should suffice. Yet in my experience a little gentle persuasion will do.
  • A dial test indicator and an adjustable mounting bracket will also be necessary.

The tools men­tioned above are essential if the job is to be carried out correctly. Yet improvisation is often the world live in.


Torque Wrenches on eBay


Dial Test Indicators on eBay


Preliminary Work

Before the differential can be dismantled and serviced it is necessary to remove the complete rear axle assembly. This is not a particularly demanding operation but you should ensure that you have adequate time to complete the job and that you have adequate working facilities, preferably under the cover of a garage.

  • Begin by disconnecting the batteries which are located under a steel panel behind the seats.
  • Loosen the rear wheel nuts or knock-on caps and jack up the car so that it can be supported on axle stands. Make sure that the axle stands are positioned so that they will not impede removal of the axle.
  • Next, mark the prop-shaft flanges so that they can be re­placed in their correct relative position to ensure that the balance of the unit is not upset, and use a pair of spanners to undo the four nuts and bolts which hold the two flanges together.
  • Undo the drain plug and allow the oil to empty from the differential casing while you remove both of the rear wheels.
  • The exhaust system must also be removed by undoing the bolts at the manifold clamp and releasing the two mounting brackets which attach the whole assembly to the underside of the car.
  • If your car is fitted with a four­ part exhaust system, it may be possible to remove only the rear silencer and link pipe by undoing the “U” bolts.
  • Remove the nuts and washers which secure the ends of each check strap to the anchor pins on the axle and lift away both of the straps.
  • Use a pair of pliers to remove the split-pins and clevis pins which retain the hand­brake cables to each brake operating lever and undo the nut and Phillips screw which hold the handbrake cable clip to the axle casing.
  • Remove the self-locking nut and the flat washer which secure the brake balance lever to the pivot on the axle casing and lift the whole cable assembly out of the way so it does not become obstructive.
  • Next, undo the nut which secures the lower end of each damper link to the bottom brackets of the leaf springs and disengage the links.
  • Place a piece of polythene over the hydraulic fluid reservoir, unscrew the hydraulic brake supply pipe union, and remove the flexible pipe from the support bracket which is located on the side of the battery box.

MGB Differential Parts on eBay


What Next

The next stage is to read and follow How To Remove Axle Shafts On An MGB


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