As title (posted this in the general section then spotted this more appropriate forum, sorry).

Back in the Cosworth world after years away, and I need to change the rear diff on my car, as frankly, it's crap. It's either a totally worn out LSD or the previous owner/clown has fitted an open diff. To be honest for all I know he's fitted a 7", I got the Cossie paranoia already.

I know the differences aren't huge between a 7 and 7.5, but know they do look different externally, but exactly the differences I don't know, and searching has found nothing.

Can anyone explain for me?

Thanks in advance.

 

7" has splined driveshafts, 7.5" has bolt on.

Alternatively, 2WD has 7.5", 4x4 has 7".

There 'Viscous' and after time they become sloppy and need 're-rating'.

Martin

 

I didn't realise it was as obvious as bolted on or splined shafts.

I know the 2wd/4wd thing, mine is 2wd but has a lot of 4wd bits and god knows what over the years as was built as a track car, so wanted to be sure.

And yes, viscous diffs arent great even new, once old they're rubbish, this one could be either open or a knackered viscous, its hard to tell.

 

That's not correct. The 7" lsds on the 4x4 also have bolt on driveshafts. In fact all cars with rear disc brakes (that includes loads of normal Sierras) have bolt on driveshafts.

The difference between the 7" and 7.5" can obviously be seen on the size, but that can be difficult when you don't have them next to each other. An other thing to look for is that the 7" diff has 2 long bolts (one at the bottom, other at the top) going through the diff to mount it in the beam. The 7.5" diff has only one 1 long bolt at the bottom and uses 2 short bolts from both sides for the top mounting point.

 

Thankyou Marc, glad someone who actually knew replied- That sounds like what someone told me before, and you've reminded me, thanks.

I'll try and take a look at that top mounting point, though might not be able to. Worth a try!

 

also 4x4 7"diffs are 100mm flanges with an M8, t40?? bolt, where the 2wd 7" are 108mm flanges with m10 t50?? bolts.
maybe someone can confirm the socket sizes.

 

I stand corrected, for some reason I thought the 4x4 was spline.

The torx on the 2WD are T50, I know that as I have taken mine out before and I remember Dojj saying the 4x4 had T40 IIRC, so as Fraser says.

Martin

 

I don't think the actual bolt sizes are different. I think they changed the head size from T40 to T45 at some point, probably cause they are that easy to strip if you are not careful.

The 108 or 100 mm flanges don't say if it's a 7" or 7.5" diff. A lot of 7.5" diffs also use 100 mm flanges. Only the 2wd cossie and Scorpio mk2 use 108 mm flanges. Although the Scorpio and 2wd cossie flanges are not interchangeable, due to the 2wd cossie being LSD and the Scorpio a normal open diff.

 

Hmm, not sure on the T40 size, but the bolts on mine are definately T45. I don't think T50 was every used though.

 

4x4 cosworth bolts are definitely a smaller diameter thread than 2wd cosworth.

 

I guess the 2wd cossie is T50 m10 bolts then. My car has a 7.5" Granada mk3 diff and has T45 m8 bolts.

 

are all 4x4 7inch diff a LSD ???

 

All 4x4 cosies had 7" lsd diffs with m8 bolts on 100mm flange and all 2wd cosies had 7.5" lsd diffs m10 bolts on 108mm flanges!
you can tell if its a 7" or 7.5 inch diff by measuring where the long bottom bolt goes through to mount it 7" bolt should be 208mm long and obviously 7.5" bolt is slightly longer

 

I can always tell by looking at the very bottom of the diff ( closest to the floor)
The bottom of the 7" is alot more rounded than the 7 1/2"
The 7 1/2" has a very flat bottom . Easy to tell at a quick glance

 

I'll take some pics or something if you can tell easy enough!

It's rubbish whichever is in it, number 1s, not number 11s!

 

Why not measure it lol, the clue is in the name - 7" is 7", 7.5" is 7.5"

To make it clear

 

Because nobody told me that is what it meant! I presumed it was the CWP size or something, and the way all the advice on here didn't mention it I guess they didn't realise either lol.

Either way, I found the ID tag, and it says...

V86BB AAA 364

Am I right in saying that's a stock 3door Cossie diff? Some French website I just translated seems to think so.

 

I was more astounded that with the amount of replies nobody had stated the easiest method of determining how to identify the diff lol

V86BB AAA 364

The 364 part of that code means you have a 3.64 ratio diff, they never made a 3.64 7" (it was 3.62) so you have the correct size/ratio for a RWD Cosworth. However I don't know if it's an LSD or not?

Some info here, although your code doesn't show up.

http://www.super7thheaven.co.uk/Sier...r_Differential

 

It is mate! It says 86BB AAA is a Sierra Cosworth diff.

Job done, phew. Cheers for the help, much appreciated