Which Rear Diff?

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randall977
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Which Rear Diff?

Post by randall977 »

I'm trying to confirm which rear diff I have on my late '82 635 which has a Getrag 265/5 CR. None of the numbers I've found seem to tie in with the ETK numbers etc. It has the numbers 33.11-1207 984 (on the Aluminium casting) and 1.208 001 F(?) on the main case. The tag has S3### but the rest has gone.

Rear-Diff.jpg
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Last edited by randall977 on Sun Feb 25, 2018 9:31 pm, edited 1 time in total.
wattsmonkey
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Re: Which Rear Diff?

Post by wattsmonkey »

Should be 3.07 lsd, centre mount medium case.
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Re: Which Rear Diff?

Post by wattsmonkey »

There would have been a metal tag attached to a rear cover bolt back in the day with key details on it.
"Most of it necessary; all of it enjoyable." LJKS
'84 635CSi, dogleg...itbs and supercharger????? Eaton Mess
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randall977
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Re: Which Rear Diff?

Post by randall977 »

Thanks for the reply, the tag has S3 but the rest has gone... I don't really want to open it up as it's not leaking, the oil is clean and it feels okay - should I look inside for any reason while it's out? I'm going to put new oil in, clean it up, repaint etc.
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Brucey
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Re: Which Rear Diff?

Post by Brucey »

it would normally be a LSD if originally paired with a 265/5 gearbox. This means you need to use a special LSD oil. It also means (especially given the sate of your gearbox) that it has likely seen a hard life and there is a fair chance that the LSD mechanism is worn and needs a rebuild. IIRC there is a test (detailed in the workshop manual?) you can do to see if the preload is correct or not.

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Ralph in Socal
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Re: Which Rear Diff?

Post by Ralph in Socal »

The torque required to break one output from the other side should be 25 ft. lbs for a healthy diff. Secure one side and find out how much torque is required to turn it separately from the other side

To determine Gear Ratio (yours ahould be 3.07 as mentioned), count how many revolutions it takes for the input to turn the output one complete turn. Very easy to do when it’s out like that.

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randall977
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Re: Which Rear Diff?

Post by randall977 »

Such a great forum - thanks!

Well it is 3.07. It takes around 45 ft. lbs to break one side from the other. Is 25 right? - that's not much force.

If it should be 25 ft. lbs, what does it mean if mine takes 45 ft. lbs to start slipping?
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hornhospital
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Re: Which Rear Diff?

Post by hornhospital »

It may mean you have a rebuilt/upgraded diff with 40% limited slip instead of the factory normal 25%. Not a bad thing!
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Re: Which Rear Diff?

Post by wattsmonkey »

randall977 wrote: Sun Feb 25, 2018 9:45 pm Thanks for the reply, the tag has S3 but the rest has gone... I don't really want to open it up as it's not leaking, the oil is clean and it feels okay - should I look inside for any reason while it's out? I'm going to put new oil in, clean it up, repaint etc.
That's cool, the "S" is the bit that confirms it's an LSD (I know you've worked this out already!), and it's quite normal for none of the tag to be legible or for it to be missing altogether.
"Most of it necessary; all of it enjoyable." LJKS
'84 635CSi, dogleg...itbs and supercharger????? Eaton Mess
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randall977
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Re: Which Rear Diff?

Post by randall977 »

Just did another test - about 50ft.lbs is needed to slip. I'm amazed to find something that works :o - good news!
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