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MG Midget and Sprite Technical - Rear Axle Rebuild

Stripping my rear axle for rebuild as I got a bit bored of bodywork. Anyone got any hints or tips on what to look out for in the rebuild?

A few pictures of what I have found so far...

1. One of the half shafts had a bit of nasty looking swarf on it... One of the few receipts I have is for a new shaft sometime in the 90s. One shaft does look newer than the other, broken shaft?!



Malcolm Le Chevalier

2. Other shaft (new one) has a bit of wear on the splines, but nothing too tragic, right?

Malcolm Le Chevalier

Gunk on drain plug didn't look promising to me...



Malcolm Le Chevalier

And gunk coming out too (note, the big chunks in the bottom is just crap that was in the tin already...

Malcolm Le Chevalier

Finally, there seemed to be a bit of front/back play in both the hubs (assuming normal?) and quite a bit of side to side play in just one of them.

Nothing will get buggered if I pull the diff this afternoon will it, as long as I don't start undoing all the bearing carriers, right? Will do that this afternoon.

Thanks for listening, thoughts and advice appreciated as always.

Malcolm
Malcolm Le Chevalier

Hi Malcolm,

hopefully this will be helpful:

http://www.mgexp.com/article/mg-midget-differential-rebuild.html

You can remove the diff from the housing, and replace the thrust washers, with simple hand tools. Doing this will nicely tighten up a worn unit, and allow you to thoroughly clean and inspect your teeth.

However, if you want to replace the pinion seal, and / or bearings, you will want to either get a 0 ~ 15 in-lb torque wrench to set the preload with, or have a local shop who specializes in differential rebuilds set it for you.

If you are not going to touch the pinion bearings, and just replace the seal, a clever technique that others have found success with is to use that same small torque wrench to measure the drag before you start, and the tighten the nut until you reach exactly that same amount of drag. It is tough, though, because the difference between, "too loose" and "too tight" is only a fraction of a nut rotation (much less than one flat), so getting it right can be hit or miss. Luckily, the crush spacers are cheap, if you need to do it again.


With as much shiny stuff in your oil as that, I doubt that your bearings are OK, and worry about the teeth. Let us know what you find!

Norm

Norm Kerr

Thanks Norm, I have read your page before with interest.

Diff is out, teeth look fine to me but I forgot to take a picture, doh!

There was a lot of grey sludge in the axle casing too :-(

Malcolm
Malcolm Le Chevalier

Also been reading your "Rear axle knowledge" page Norm, more great advice there thanks.

I tip my cap to you sir!

Malcolm
Malcolm Le Chevalier

I just tried to coalesce all of the knowledge that was trapped here, in the archives, into one place. I spent a lot of time reading all of the past threads and took lots of notes. Then, read (and re-read) the Bentley, and the Haynes, and then did some research on differentials from other RWD cars, like Mustangs and Camaros, and found that the similarities were way more than the differences. Probably there are ways to make it better, and I am always open to suggestions!

"Pay it forward" :)

Norm
Norm Kerr

Diff looks OK to me?

Where do they typically wear, I have pics of all the different gears. Or is it bearings wearing that cause problems?

Malcolm

Malcolm Le Chevalier

Hi,
It can be difficult sometimes to capture in a photo, but what you want to look for is any change in the surface quality of the tooth, across its face. There will be some color difference where the wear has been occurring, and that is normal. What you don't want to see is a change in gloss level.

A dull area (like what you see in a valve seat after lapping your valves) is bad on a gear tooth.

If the teeth all look good, then feel the bearings as you rotate them slowly with your fingers (wash the bearings real well with solvent first, so they are clean, and fairly free of oil). If they feel smooth, with no catching or "grinding", then take a look at their races, and the balls (as much as you can see, with a bright light). If they all look equally glossy, with no dull spots, then you should be fine to re-use them.


The most common wear item in an otherwise healthy differential is the thrust washers, which are easy to replace once you have removed the little pin that holds the cross pin in the cage. On an MGB that can be done without even removing the diff from the axle housing, but on a Midget, the crown wheel must be first removed, to get access to it, and then a really long center punch used to push out the little pin. The rest is straightforward (be sure to return the little gears in their original orientation to each other when you re-assemble).


Norm
Norm Kerr

This thread was discussed between 05/10/2013 and 06/10/2013

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