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MG MGB Technical - Tips for assessing the differential

Hi,

I've a salisbury axle of which I know very little about. I was thinking of perhaps performing a minor overhaul thrust washers, bearing seals etc. to be on the safe side.

I have been doing the pinion flange test - looking for clunks, and I've been finding it difficult to find any notchiness/clunk. After rotating anti-clockwise, a very slight click about 4mm into a clockwise turn at the pinion flange. It is on the whole quite a smooth pick up.

Are there other tests to help me assess the condition of the diff?

Also, a) are there pointers as to condition of bearing seals apart from traces of leaks at the brake drum backplates.

b) would you replace if the even if the seals seem ok?

Ta

Iwan Jones

Consider the old saying IF IT AIN'T BROKE DON'T FIX IT UNTIL IT IS.
Sandy
3:0:5^110:0:5^127:0:2

Consider the old saying IF IT AIN'T BROKE DON'T FIX IT UNTIL IT IS.
Sandy
3:0:5^110:0:5^127:0:2

X2,

X2!
Michael Beswick

IF all your'e getting is 4mm "free rotation" at the flange, leave well alone, hell it sounds NEW!!!
Allan Reeling

The real arbiter is if it whines when running, the note changing as you go from throttle on to throttle off. Even then it is only an annoyance, it will carry on like that for generations.
Paul Hunt

Diff backlash due to worn thrusts is one thing that is definitely a stitch-in-time thing. Once the thrusts have worn completely, the cage is next and repair/replacement of that can be costly, whereas the thrusts are dirt cheap.

I measure the lash at the wheel because I'm a lazy fella and qon't get under the car unless I need to. Put the car in first gear and jack up one of the back wheels, leaving the other on the ground. measure the lash at the periphery of the wheel using a reference point on the body.
15mm or less, good.
25mm moderate thrust wear.
25-40mm, time to do something about it.
50mm the thrusts have probably just gone or are about to. Fix it ASAP.
More than that, start saving!
Paul Walbran

Paul,
thanks for your very useful information for checking backlash on the differential. I have checked mine today as you have recommended and measured 16mm, so I guess that is one thing less to worry about :-) A 1976 car with 96K miles ( not sure what PO have done, but seems well within tolerance).

Thanks again

Andy
Andy Robinson

Generally the only thing that wears the thrusts is wheelspin. Of which the many cars received a lot in their young days, in the hands of young drivers ( = many of us at the time :-))
Paul Walbran

Hi,

Many thanks to you all. However, I have a further question or two. How do you measure backlash at the wheel with the axle on a bench? What exactly am I looking for - in very simple terms for me ..... not done anything on diffs before.

Ta,

Iwan
Iwan Jones

The backlash is measured at the ring gear with a dial indicator gauge and magnetic base. Essentially, the pinion gear is locked in place while the foot of the dial indicator gauge is placed on the edge of one of the ring gear's teeth. With the ring gear moved all the way forward until it stops, the dial gauge is set to zero. Then, you move the ring gear in the opposite direction and read how much the needle has moved. This is the backlash reading. RAY
rjm RAY

There are two separate references to backlash above. Ray describes the backlash between crown wheel (ring gear) and pinion, my earlier post describing backlash at the wheel is referring to the total backlash in the whole assembly from all sources, including that of the crown wheel and pinion plus any play in the centre differential gears and any movement in the splines between halfshafts and side gears.

Generally, if the diff is operating without a whine at all, the backlash betwen crown wheel and pinion is OK and any excessive lash at the wheel will be down to wear in the thrusts.

If you have the diff on the bench, what you are looking for is any play in the centre differential gears. If there is, you should renew the thrust washers behind them. Whilst doing so, check the condition of the cage they run in - if the thrusts have completely worn out now or in the past the cage could be badly worn. Usually this is under the small pinions if it happens, but I have seen the cage flogged out under the side gears as well.

If it has worn, the best solution is to fit another unworn cage, but this requires specialised work getting the crown wheel-pinion backlash correct so isn't a simple swap. There are a number of other cheaper and easier fixes I have seen, including two thrusts behind each small pinion, or oversize thrusts under the side gears. They work to verying extents and whilst not ideal they are better than nothing.
Paul Walbran

This thread was discussed between 04/11/2014 and 08/11/2014

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