How To Position The Differential Pinion On An MGB

How to position the differential pinion 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 – Position the Differential Pinion

The setting of the pinion position in relation to the crown wheel is a critical part of the overhaul procedure and much care should be taken over this step.

Their relative positions are determined by spacers which are available in various sizes.

IMPORTANT : Now I am not convinced this ‘bygone’ guide provides a clear explanation below yet does address some of the particulars of an MGB differential. I also think the guide is missing one crucial picture to help explain. Therefore I have hunted down this youtube video which does help explain the objectives.

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

Begin by carefully drifting the bearing outer races into the axle casing. This was originally done with a BL dummy pinion (18G 191 H) which was designed to accom­modate a dial indicator gauge.

IF YOU HAVE NO DUMMY PINION, WHICH IS MOST LIKELY THE CASE:

The following, until after next picture, is the original thinking but today you will need to address this stage with your pinion and using a dial gauge to measure the average the relative position of the pinion to both of the bearing races and then take an average of the difference to consider its correct position and spacer needs.

Use the dummy pinion and fit the inner race of the bearing but do not use the collap­sible spacer and the oil seal and note that the standard pinion head spacer is already incorporated in the shank of the dummy pinion.

Install the universal-joint (UJ) driving flange by tapping it into place with a soft-faced hammer and tighten the nut a little at a time until you achieve a bearing pre-load figure of 0.11 to 0.2 kg/m (10 to 20lb/in).

Original advice: The pre-load of the bearing is the force which is required to rotate the pinion in the bearing and this friction can be measured either by using BL service tool 18G 207 or by hooking a spring balance into one of the flange bolt holes, but since these holes are 38 mm (l.5ins) from the pinion shaft axis, a resistance of 0.11 kg/m (l0lb/in) is the optimum pre-load figure using this method.

Alternative: Use a rotational torque measuring device to measure and set the bearing preload shown in the attached above you tube link.

Next fit the dial indicator to the base of the dummy pinion shaft and zero it.

Move the indicator arm so that the probe rests on the centre of one of the differen­tial bearing bores and, after locking the dial indi­cator mounting bracket in place, obtain the maximum depth reading and make a note of any variations from the zero measurement mark.

Another way of achieving this measurement is shown in the picture below.

Position a Pinion on an MGB
The pinion depth can be measured also by mounting the gauge in this way and then subtracting the depth of the bearing bore

Repeat this procedure for the other bearing bore and then calculate the average variation by adding the two numbers together and then dividing them by two. This reading will represent the amount of correction which is required to adjust the standard spacer which is 5.3 mm (0.208ins) thick.

If the dial indicator has given a negative reading, the spacer must be reduced by the amount of the reading, but if the figure is positive, the thickness of the spacer must be increased by the amount indicated.

However, a final calculation must be made, since allowance has to be made for the mounting distance which is engraved on the head of the pinion to be fitted; this number is enclosed in a rectangular bracket.

If the number is a positive amount, reduce the thickness of the proposed spacer by the amount indicated on the pinion head and if the number is negative increase the spacer thickness by the appropriate amount.

It is essential to take great care over this step since a tolerance of only 0.025 mm (.00lins) is permissible in the thickness of the spacer which is finally fitted.

Compute the size of the spacer required, by methodically performing a written calculation of the type used in the example below, to set the crown wheel position.

MGB Differential Pinion Positioning Calculations

Next it will be necessary to fit your pinion so that the correct pre-load figure is achieved.

First remove the dum­my pinion and fit the spacer which has been selected to the pinion.

Next fit the pinion inner bearing race and insert the pinion shaft into the case.

Slide on the collapsible spacer with the smaller diameter towards the shoulder of the pinion head then press the outer bearing on to the pinion, taking care not to compress the collapsible spacer.

Grease the pinion seal and drift it home so that it is flush with the axle casing and then fit the universal-joint (UJ) flange together with its nut and washer.

Before the pre-load can be adjusted by progressively increasing the tightness of the flange nut, an initial torque figure of 20. 7 kg/m (140lb/ft) must be achieved by holding the flange stationary.

After torquing the nut to this initial figure, the torque should be increased slowly by l kg/m (l0lb/ft) so that the spacer starts to collapse.

Rotate the flange several times to allow the bearings to settle and then measure the pre-load using one of the methods outlined above.

Progressively tighten the nut a very slight amount at a time until the pre­load measured is 0.10 to 0.16 kg/m (14 to 18lb/in).

How­ever take great care to increase the torque only by the smallest amount possible between each pre-load check, since the bearing pre-load can be reached very quickly and if a pre-load of 0.3 kg/m (24lb/in) is exceeded, the pinion shaft assembly must again be dismantled so that a new collapsible spacer can be fitted.


What Next ?

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


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