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MG Midget and Sprite Technical - Rear Axle Whine

I recently swapped the internals from my rear axle to a refurbed casing due to rust holes in the original's back plate.
As part of this I replaced the hub bearings but everything else (apart from seals and gaskets) was swapped straight across. I didn't undo the pinion nut etc or 'mess about with' the diff.
Now there is a loud whining from the rear axle when driving. It is there at all times when the axle is being driven but disappears when the accelerator is lifted ie on 'overrun'.
Has anyone got any suggestions as to what might have caused this? Is it likely to be due to a slight variation in the axle casing dimensions or something else I may have misaligned? Any suggestions as to what I could check or adjust to eliminate it.

Car is 1972 model.

Thanks,
Dave
Dave Thrussell

Did it happen as soon as you drove it with the new casing, or did it happen soon after?

What oil are you using?
Dave O'Neill 2

Hi Dave,

I honestly can't remember if it was there right from the start - but it was definitely apparent on the first decent (higher speed) run I did.

Running with EP90 in the axle.

Cheers,
Dave
Dave Thrussell

Casings can become misaligned due to heavy impact eg accident damage or even with the kerb in extreme cases. It means the half shaft runs slightly out of line and can cause a noise. I experienced this when I had an axle overhauled by Mallock racing and they used a long straight bar to check the alignment of the axle tubes.
Only an explanation I'm afraid, no easy answer.
Bob Beaumont

When you replaced the bearings, how did you shim them?
check this out:
http://vintage.mitchell1.com/PClubData/chassis/enis75/V2I757070.pdf
D mckellar

DM - he's changed the hub bearings, not any of the bearings in the diff itself, so no shims needed.
David Smith

Our old friend Nigel recommended a synthetic oil i remember, said it worked wonders on his diff....

Mine used to have a whine but it was basically dry after many years of not being used, maybe the oil takes time to get through to where it should be ?

I put 120 weight in mine and its nice and quiet now
Andy Phillips (frankenfrog)

Hi Dave,
Did you change the diff oil? You MUST use "hypoid" oil in a Spridget diff. I know someone who used a non-hypoid oil in a Spridget diff (fitted to a race TC) and every lap the whining became louder. The pinion was worn out in a few laps!
Bob Schapel
R L Schapel

You might think this odd but have you removed the spare wheel from the boot & not replaced it?
Gavin Rowlesx

Thanks all for your comments, interest and suggestions.

I can't remember the spec of the oil I used when refilling the diff - will check tomorrow when I go in the garage. If it isn't "hypoid" i'll try changing it as this would be the simplest remedy. Unfortunately I think it will be the same as I was using pre axle rebuild so not sure how much difference it will make.

Gavin - I see where you're coming from, I believe they had that problem at the factory at one point on the test drives before they put the spares in the freshly built cars. But I haven't taken anything out of the boot.

Cheers,
Dave
Dave Thrussell

Sorry to say that if the oil you are using is not "hypoid" the damage would already be done. Changing to hypoid oil would help prevent further wear but would not fix the problem. Lets hope that the oil is not the problem.
Bob Schapel
R L Schapel

EP90 is OK for hypoid gears, isn't it?
Nick and Cherry Scoop

Yes it is;
"Because of the sliding contact that hypoid gears make, their hydrodynamic contact pressure is higher. To be suitable for use with hypoid gears, a lubricant must be capable of resisting high pressures.
Oils with "EP" ratings (Extreme Pressure) such as EP90 are required. Some brands describe themselves as 'hypoid' instead, a term which is synonymous with EP"
David Smith

Just make sure it is EP90 and not just 90 weight gear oil.
Dave O'Neill 2

This thread was discussed between 14/03/2016 and 16/03/2016

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