How To Examine Differential Components On An MGB

How To Examine MGB Differential Components

This guide captures ‘bygone’ advice on how to dismantle and examine differential components on an MGB

It is part of my series on MGB Differential Rebuild.

Go here for all my Classic Car Maintenance DIY Guides or visit SCOTTYS Technical Library and Supplier Library 

Useful information for Classic Car and Retro Car enthusiasts, as many of these tasks are no longer required on modern day cars.

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





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

Step By Step – Dismantle The Differential

Begin by removing the universal­ joint (UJ) flange retaining nut using a socket wrench and a tool which will prevent the flange itself from rotating.

There is a BL tool manufactured for this job (18G 34 A) but it is easy to fabricate a tool and improvise.

Remove the UJ Joint Flange on an MGB Differential
Remove the UJ flange retaining nut using a socket wrench and a tool which will stop the flange from rotating

The pinion flange can now be tapped out gently using a soft-faced mallet, and its oil seal can be prised out carefully with a screwdriver and discarded.

Now remove all the crown wheel retaining bolts.

The crown wheel can then be removed by tapping it off the differential carrier with a soft­faced mallet. Drift out the concealed roll pin which locates the pinion pin and then withdraw the pinion pin from the casing

Remove Pinion Roll PIn on MGB Differential
Use a drift or a suitable nail to drive out the roll pin which locates the pinion pin in the casting of the carrier
Remove Pinion PIn on MGB Differential
Now use a larger drift to drive out the pinion pin. but make a note of the way in which the pin must be re-fitted

Now rotate the differential wheels until the pinions are opposite the apertures in the differential cage (first pic below) and lift out the pinions together with the thrust washers which are located behind them (second pic below).

Dismantling an MGB Differential
Rotate the differential wheels until the pinions are opposite the apertures in the differential cage
Thrust Washer Removal on MGB Differential
The two pinions can be lifted out together with their thrust washers. Store each pair of pinions and washers separately

The differential wheels can then also be lifted out, together with their thrust washers. Take care that none of these components are interchanged and make a note of their relative positions.

Removing Differential Wheels on an MGB
The differential wheels and their thrust washers can also be lifted out. Check the washers for signs of cracking

The differential inner bearing races can be removed using a two-legged bearing puller (originally achieved using or BL service tool 18G 47 C, together with adaptor 18G 47 AK and plug 18G 47 AR)

Removing Differential Bearings on an MGB
The differential bearings can be removed either with a puller or by tapping them off with a hammer and a drift

Fit the tool to the differential assembly so that you can clamp the protruding lip of the tool’s flange firmly in a vice. Have an assistant hold the differential assembly while the handle of the tool is turned clockwise and the differential cage is pushed out of each inner bearing race.

Alternatively, it is possible to drive the bearing races off using a hammer and a drift but great care should be taken not to damage any of the components.

Finally, remove the pinion shaft bearing in a large press.

Remove Differential Pinion Shaft Bearing on an MGB
The pinion shaft bearing is an interference fit and can only be removed by using a high-pressure hydraulic press, although I do know some who have achieved this at home.

Step By Step – Examine The Differential

A thorough examination of all the differential components should be made after cleaning them in a proprietary de­greasant such as Pioneer or Gunk, in order to determine which parts require replacement.

Check all bearing rollers and races for wear or scoring and purchase replacements for any bearings which appear to be suspect.

It is generally a good idea to replace all the bearings while the unit is dismantled, since they are relatively cheap and it is false economy to compromise on repairs of this nature.

The teeth of the crown wheel and pinion should be checked carefully for general wear and particularly for pitting, scoring and chipping.

These two components must be replaced as a factory-matched pair if they appear to be defective.

Check the teeth of a crown wheel on an MGB
Check the teeth of the crown wheel for pitting or other damage. This one has worn badly and must be replaced

The differential wheels should be checked for similar signs of wear and all thrust washers should be examined for scoring or pitting.

A new pinion oil seal must be fitted at the re-assembly stage and so must a new col­lapsible pinion spacer.

Differential collapsable pinion spacer on an MGB
A new collapsible pi n1on spacer must always be fitted at the re-assembly stage. The smaller end faces the pinion head

To complete this overhaul it is also advisable to replace the axle-shaft seals and bear­ings as a matter of course.


What Next

Head back to MGB Differential Rebuild to select the next stage instructions.


MGB Diff Parts on eBay


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