M6 / M635 Diff Housing

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Boggie
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M6 / M635 Diff Housing

Post by Boggie »

Hi All,

Looking for a RHD EU E24 M635CSI diff casing or complete diff please, I think...

The car has been in a barn for two decades and the rear diff housing (not to mention exhaust) is a bit rough. Some of the external case webs are really flakey and rusty. If only this had been an old Brittish car, like my TR3a and S2 Landy, the engine and gearbox oil leaks would have rustproofed the underside and I would not have this problem... :)

My M635 has no diff oil leaks and I could potentially carefully remove any significant rust, treat the case with Dinitrol and coat with POR15 to stabilise the rest. However should I be worried about the case strength / integrity in the long term? I am restoring this car properly, so it will last many more years. I need to get this right.

Buying / fitting a relatively low-mileage replacement (if I can find one) may well be best option OR am I being paranoid and even with some webbing deterioration the case will be fine? I am assuming the case is cast iron (it looks like it) rather than steel where I could replace the webs. Other than that, is there anyone in the UK who can restore webs on an old cast iron case?

Thanks
Ian
1985 M635CSI
Classic Car and Bike Collector & Restorer.
brickwhite
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Re: M6 / M635 Diff Housing

Post by brickwhite »

the rear diff is cast iron, it should be fine unless its whining while you take it for a spin.
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wattsmonkey
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Re: M6 / M635 Diff Housing

Post by wattsmonkey »

I think you'll struggle to find a replacement, I am pretty sure it's only the E28 M5 and E24 M635 that have the 3.73:1 LSD.

The large case housing will be the same as all the post-'83 635s though.

I bought the internals from an M635 diff with the intention of rebuilding them into a good case that I've got in the spares stash, but this isn't going to happen any time soon! The internals were for sale because the case had rotten away, so they can get pretty bad.

To be honest, I wouldn't worry too much about the webbing - there is a lot of meat on that casting!

Cheers,

Rob
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'84 635CSi, dogleg...itbs and supercharger????? Eaton Mess
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Boggie
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Re: M6 / M635 Diff Housing

Post by Boggie »

Thanks Chaps,

I will remove the diff for a better look but it does only seem to have affected a couple of the larger webs and it appears they may be for cooling rather than strength. Couple of questions please:

Any hints/tricks to removing the diff or is it a straightforward job?
From the above; I can use a standard E24 to swap internals. Any Special tools required?

Cheers,
Ian
1985 M635CSI
Classic Car and Bike Collector & Restorer.
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cgswift
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Re: M6 / M635 Diff Housing

Post by cgswift »

Ian,
Would it be possible to obtain a picture of the rusted webbing? I can't imagine a cast iron diff with rust of a nature to compromise the structural integrity of the casting. Removal and media blasting would be my actions to further assess the situation; I seriously doubt that you have any issues other than cosmetic. Toph
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Re: M6 / M635 Diff Housing

Post by wattsmonkey »

No special tools required, but it is (as I'm sure you realise) chuffing heavy!

There is a knack to getting it back in, which I think involves putting the top bolts in before the lower ones, otherwise it takes forever to line everything up.

There's a great write up on mye28.com on rebuilding a medium case diff which details the same steps, the difficulty is in setting up the preload (crush tube on the pinion) and then messing around with shims to get the backlash sorted, but most of it is the same for any diff rebuild.

Good luck with this - I'm sure I'm not alone in hoping you'll post some pictures if you go down the rebuild route.

Cheers,

Rob
"Most of it necessary; all of it enjoyable." LJKS
'84 635CSi, dogleg...itbs and supercharger????? Eaton Mess
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Brucey
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Re: M6 / M635 Diff Housing

Post by Brucey »

I have seen cast iron diff casings that look like they have been used as boat anchors (covered in very thick flakey rust) and I have not ever heard of one breaking in normal service; they are substantially over-engineered. I would suggest that you just clean it up and use it as it is (if the LSD preload is still correct). In the highly unlikely event of the case cracking you will soon see the oil leak and you can rebuild it in a new case then.

I do not recommend trying to weld a cast iron diff case; this is liable to be an expensive waste of time.

BTW the rusting always looks much worse than it actually is, because the rust you remove contains (by volume) only a tiny amount of iron. You can have rust on the surface of cast iron that is 5-10mm thick and you might have lost 0.25mm of the actual cast iron.

cheers
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