How To Adjust Crown Wheel Position On An MGB

How to Adjust Crown Wheel Position On An MGB

This guide captures ‘bygone’ advice on how to position the differential pinion 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 – Adjust Crown Wheel Position On An MGB

Begin by drifting on each inner and outer bearing race to the differential.

The correct figure for the backlash, or movement between the gear teeth, to be allowed for any individual crown wheel and pinion set is etched on the rear face of the crown wheel itself.

In order to locate the crown wheel and differential assembly precisely in the bearing bores it will be necessary to calculate the size of the shims which are required between the bearings and the casing.

This was historically achieved by using a BL crown wheel setting jig. (the BL service tools were 18G 191, 18G 191 F and 18G 191 J)

IMPORTANT : It is unlikely you will have these jig tools. Therefore I have hunted down this youtube video which does help explain the objectives. Yet below this post continues with the original thinking and information particular to an MGB, which is helpful.

If it does not load automatically then try this entering this https://www.youtube.com/watch?v=dAqAqODmcj4

Begin by placing the unit in the jig and spinning the crown wheel to settle the bearings. The standard measure­ment between the bearing bores is 183.98 mm (7.243ins), but any excess which has been machined from the bores is marked A and B on the axle casing and these tolerances, when added to the standard measurement, determine the overall dimension

If the letter “A” is stamped on the casing, a further 0.025 mm (0.00lins) should be compen­sated for, while for the letter “B” the figure for compensa­tion is 0.050 mm (0.002ins).

Ensure that the differential assembly and the jig are both scrupulously clean and then fit the dial indicator with the probe positioned on the pillar head of the jig and calibrated to read zero.

The height of the jig’s pillar is the same distance as the standard height of the complete differential assembly which is 177.10 mm (6.972ins).

The height of the jig can then be compared with the actual height of the differential assembly by moving the probe of the indicator to the machined face of the jig.

Read the gauge to determine the excess distance and add this figure to the height of the bearing caps and torque the retaining bolts to 6.9 to 7.6 kg/m (50 to 55lb/ft). Attach the dial indicator to the axle casing flange as shown in figure 23 and carefully measure the backlash which should be a movement of 0.051 mm (0.002in).

If such a reading is not obtained it will be neces­sary to re-check the above procedures and to make sure that the correct shims have been fitted.

The re-assembly can pro­ceed by reversing the dismantling procedure. Found by visiting the starting post  MGB Differential Rebuild

Always take care to keep all components absolutely clean during the re-assembly steps.

When fitting the axle case cover gasket use jointing compound so that an oil-tight seal is ach­ieved.

Torque the axle-shaft nuts to 20.6 kg/m (150lb/ft).

When installing the axle assembly in the car, check the condition of the brake linings before the car is road tested.

Fill the rear axle with 0.85 litres (1.5 pints) of SAE 90 Hypoid oil having checked that the drain plug is tight.

Do not forget to tighten the filler plug.

Finally, test the car on the road.

If the work has been carried out properly, you should notice a great reduction in the rear axle noise level even under heavy acceleration.


What Next ?

This is my last post in my series Bygone Guide to MGB Differential Rebuild , head back to the beginning if you didn’t find the information you need.


MGB Differential Pinion Components On eBay

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