Identifying Sierra Rear End

[Guest evildave]

So I want to stick an LSD in but not sure what size my sierra rear is 7" or 7.5"? I think I have a 3.62:1 but will be heading closer to 4:1 if I can now that I'm running on 16" wheels.

Is there a way of telling what it is by looking at the outside? Are there drive shaft differences as well I need to be careful of?

 

Cheers

[richyb66]

Push in driveshafts (no visible bolts) at the diff end and it'll be 7" (could actually be a 6.5" but for the purposes of swapping they're the same externally).

 

Bolt on shafts at the diff end can be 7" or 7.5" in which case if the top fixing that goes across the diff is a single long bolt it's 7" or 2 separate bolts will be 7.5". If you measure across the diff casing at this point, a 7.5" diff is 195mm and a 7" is about 185mm. The 7" and 7.5" designations are the crownwheel size, not the casing width.

 

Bolt on driveshafts will be 100mm or 108mm. The bigger joints are 2WD Cossy Sierra and Cossy Scorpio. The 108mm joints are easy to get new, the shafts aren't and second hand ones are rare and pricey. 100mm shafts are probably preferred, all new parts are available and second hand parts from 4wd Cossy and bigger power Sierras are easy to find.

 

7.5" diffs only come with drum brakes and the suspension arms are slightly different with 108mm joints (more clearance required).

 

Best advice is to buy a complete rear beam if you want discs and an lsd, that way you can see everything is compatible. Buy separate and it'll cost more - ask me how I know. I run a 7.5" Cossy Scorpio diff with 108mm Cossy shafts.

 

Last of all, Scorpio beam is the same width as a Sierra one but has 5 stud wheels. 4 stud hubs for 108mm shafts are almost impossible to get hold of.

Edited June 2, 2017 by richyb66

[Guest evildave]

That's a lot of information! Excellent, thanks for the reply

[Guest lotusPaul]

7.5" diffs only come with drum brakes

 

Is this true??

[Guest Tim Norman]

No

[richyb66]

My bad, 7.5" only come with discs.

[Guest Tim Norman]

Just a little hi jacking here.

My 7.5" 3.62 slipper is geared wrong for the V8. at 70mph it is doing 3000 rpm.

Can I fit a different crown wheel to it or is it more complicated than that?

[richyb66]

Yes, but the tallest cwp for 7.5" seems to be 3.36.

 

I'm going for a taller 5th gear first, I got a 2wd R380 box that I'm going to fit shortly.

[Guest Tim Norman]

Git!

I have been looking for one of those for months, is it the one with the 0.63 5th?

Was thinking Tremec T5 until I saw the prices!

Edited June 11, 2017 by Tim Norman

[richyb66]

I don't know until I strip it. It's a 72A, L suffix which I've not been able to identify. Could be Morgan. MGRV8, Marcos??

[Snapperpaul]

Crown wheel and pinion are matched pairs, you can't just fit a new crown wheel

It's cheaper and easier to fit the whole diff

 

The only Sierra that had a 3.9 LSD was the 2.0L DOHC 4x4

[richyb66]

3.9 is way too low with a V8. I think Tim is running 3.64 like me. 3000 rpm is about 70mph which it'll do, but it would stand a much taller diff.

[Guest Tim Norman]

the 3.1 only drops the revs down to 2600 at 70 so no good anyway.

Might have to be the R380 or the T5 if I win the lottery.

[richyb66]

I'm still looking at a taller fifth gear set for the LT77. Got 3 boxes in pieces and have a selection of 5th gears. Will most likely fit the R380 then sell the others.

[Guest Tim Norman]

DOn't think you can get a taller 5th for the LT77