98
Back axle modification - possibly the most important note yet for the MGBV8!
This modification has been jointly developed by Geoff Seaton and Peter Laidler and both need no introduction as they are regular beavers for our members. The note is prepared by Geoff Seaton. See also Note 72 by Chris Hall.

We are not over exaggerating the point when we say that the following information could possibly save you a seriously damaged rear axle which might need to be scrapped.

The crown wheel and pinion for the V8 rear axle is now NLA and you could have a long search for a suitable replacement or have the parts made at considerable expense. It is these parts which make the V8 back axle a special part, along with the differential cage of course. It goes without saying that if you ruin these two parts then you are in deep trouble. Several times over the last few months, Peter Laidler has had members asking him where can they get a CWP as the axle has smashed itself apart. Chris Hall had a lucky escape as he explained in his earlier workshop note when the roll-pin sheared off in his axle.

Inside the differential cage (BTB840) there is a pinion pin (BTB871) that holds the two pinion differential wheels (BTB1249) and the trust washers (IG7445). The pinion pin is held in the differential cage by a small roll-pin (BTB715) which stops the pinion pin from rotating and falling out. These parts and an exploded view can be seen on pages F11 and F12 in the BL parts list and as items 13, 17, 16 and 18 respectively on page Ha3 in the workshop manual.

What has happened recently in all the cases reported recently is that the small roll-pin (BTB715) has somehow lost its spring tension and dropped out or it has sheared, allowing the hard pinion pin (BTB871) to fall out of the differential cage while the wheels are rotating. The first thing this very hard and thick pinion pin does is to hit the main axle drive pinion and smash it - and possibly the axle case and differential cage as well.

What can you do about this! Well the first thing to do is to go out now and buy a new roll-pin (BTB715) and a couple of new pinion wheel thrust washers and a split pin about two inches long with an outside diameter which will allow it to go through the centre of the roll-pin. A 7/64" (seven sixty fourths of inch) pin is a perfect fit.

Before any stripping down action, clean the axle and the surrounding area before the removal of the axle cover plate. Drain down the rear axle oil and remove the axle case cover and, with the axle on axle stands, rotate the wheels until the small roll-pin can be seen at the top or bottom of the differential, cage. Drift the old roll-pin out and with a soft drift, tap out the pinion pin until the roll-pin is visible. Then remove the two pinion differential wheels and their thrust washers.
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Clean up and replace the wheels. Add the new thrust washers, then slide the pinion pin back into place, align the holes and carefully punch the new roll-pin into place. Now insert the small split pin through the hole in the centre and spread the ends out in the normal manner.


You can see now that you have effectively doubled the roll-pin's sheer strength as it is now virtually solid and if it should lose its tension with age, then it will not slip out. To make doubly sure, we would recommend reforming the eye of the split pin by spreading it slightly, although if the correct size of split pin is used, there is no danger of it passing through the differential cage bore. You might question whether there is space available for the head of the split pin and the bent over legs. Do not worry, there is ample space and it will not foul the differential. Before refitting the axle cover plate, fit a new gasket (BTB674).

As a result of changing the thrust washers you might be pleasantly surprised to find that the niggling little "clunk" has reduced or possibly disappeared. Without going into the technicalities of the differential, these two thrust washers play a very important part in the drive from the pinion wheel to the half shaft wheel. As these are straight cut tapered gears, there is a natural tendency for them to "run out" and with worn thrust washers, the pitch line through the gears will become badly aligned. We found signs of wear and grooving on Peter Laidler's V8.

To make this modification as simple and inexpensive as possible, and bearing in mind that these thrust washers and roll pins only come from BL in packets of ten, some kits have been made up. The kit contains two new thrust washers, a new roll pin and a split pin and the kit costs £1.75 inclusive of post and packaging! I am grateful to Peter Laidler for his assistance in preparing this note.

This modification is equally important for MGC owners as that car has similar NLA parts. It also applies to the MGB 1800 but the parts are still available but costly. Remember now is the time to do this modification - tomorrow might be too late. If you feel the modification is a little too complicated for you, then take the V8 with a back axle life saver kit to a reliable V8 specialist like Geoff Allen or Brown and Gammons and get them to do it for you.

Regarding lubricants, I am not a lover of additives which contain solids but in the case of the back axle I do think Molyslip Transmission oil added to your favourite Hypoy gear oil may not be a bad thing. This additive definitely reduces friction and hence the running temperature of the component. The correct amount of Molyslip is one fluid ounce to each pint of gear oil. Finally do remember the rear axle oil on the V8 is not the same as the gearbox oil as the V8 rear axle has Hypoy 90 containing a high load additive.
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