I have a bit of diff confusion. I'm using a Fireblade motor that requires a numerically low diff. In the US, we are a bit limited in our selection. I did a bit of research and found that the Mercury Merkur in the US uses a 7.5 " 3.64 diff. I bought one for a small sum and took it home. Admittedly, I didn't think too much about it. Now I need to start looking into buying custom halfshafts, and before I make a costly mistake, I have this question:

What is the difference between the 7.5" that I have and the 7" many of you use? To me, they both look the same. Is it possible to put a 7" LSD in a 7.5" diff?

But alas, here is the big question. If, later in the build, I want to import the 7" LSD, will I have that option or will I have to have new halfshafts made.

--Thanks, Chris

I presume you're talking about an independent suspension setup derived from the Sierra in the UK ...

The 7.5" diff in the UK is normally fitted to Cosworth Sierra's and is an LSD (viscous type). The difference is in the crownwheel and pinion in that the teeth are much deeper requiring the diff case to be a half inch wider. The drive shafts are bolt on type. The contents cannot be transplanted to a 7" casing.

In the UK, standard Sierra's use 7" casings. The XR4, XR4x4 and XR4i mostly have an LSD within this casing and is the most common LSD from this type of car to be used.

If you swap from a 7.5" to a 7" diff the (UK) drivers side will stay the same length (the longest shaft) but the (UK) passenger side will need to be a half inch longer .. There's not enough give in the driveshaft CV joints to take this up.

nice info dude. So are all 7.5 inch diffs LSD? cheers.

Mine is not. I can hold one of the 'ears' and still turn the other. Is it hard to source an LSD for the cossies?

And you are right: my diff is not symetrical; one side does stick out a bit more than the other.

--Chris

Joel,

I have a 7.5 inch LSD and a 7.5 inch NON LSD so the answer to your question is no

7.5 inch ford rear if its the same guts as the rest of fords NorthAmerican rears live or not are pretty much swappable inards from one rear to the other. Right know I am searching for a 7.5 lsd pod to put in my rear axle. Some ford rangers, bronco11 and aerostar vans and 6 banger mustangs had the lsd. Pretty sure the gear range can go from 2.85ish to 4.11

Dale

sierras i have heard off range from 3.11 to 3.92 i think. are there others? or is the 7.5 inch fitted to stuff like granadas too?

ps cheers for the correction guys.

Are you using a live axle or a 7.5” Merkur diff? I am interested in following how you will swap a North American LSD into the 7.5” diff. If this works, I could save a small fortune over sourcing/posting one from the UK—assuming it is of the same quality.

--Chris

In my car I can fit either diff.

The external differences are as follows:

The top mounts are further apart on the 7 1/2".

The diff flange is further forward on the 7 1/2" diff by about 5 to 7 mm.

On my diffs the output flanges on the diffs are the same distance apart meaning I can use the same drive shafts.

I have centralised my diff so I use equel length drive shafts on each side.

John

Can anyone post a picture of an standard ford LSD as used in the Cosworth or XR's.
If not does anyone know of any sites that shows one out of its housing?
I have an 7" diff believed to be an LSD, but on removal of the backplate it looks very similar to an standard diff, FatChapChipChop mentiones an "viscous type" has anyone seen one of these that could decribe it?
Thanks

Doh, just got myself a Cosworth rear diff for the bargain price of £40, didn't realise the driveshafts were different

Just to clear it up more, the only cosworth sierra which ran the 7.5" diff was the 2wheel drive. The 4x4 sierra cossy used the same 7" as the xr4x4.

If its a 7.5 your after, also check out the early (pre 94 IIRC) Scorpio Cossy as again it used the larger of the two LSDs. After that date they used a non LSD and traction control.

Rgds Rich

[Edited on 7/12/05 by RichieC]

quote:

---

Originally posted by MikeRJ
Doh, just got myself a Cosworth rear diff for the bargain price of £40, didn't realise the driveshafts were different

---

Don't know to be honest, it was bought through a friend. I have a standard diff from a 2.0L Pinto engine sierra burried in the back of my basement, will drag it out and compare.

EDIT: Just spoke to said friend to assures me it was a 2WD car, so will need to source the driveshafts, or at least the different length one. Where these driveshafts used only on the Sierra, or did the Granada also have them?

[Edited on 8/12/05 by MikeRJ]

Hello...

like JB says, i think the output flanges are the same distance apart on the 7 and 7.5 diffs. If they're not then I would have thought 1/2" can easily be taken out with movement in the shaft - each shaft has about 40mm of travel.

Even with that news, MikeRJ, I'm afraid that if you really have a RWD cossie 7.5" LSD you need to find two shafts anyway - the RWD cossie shafts are big fat bad boys with a larger flanges than all the other applications (108mm instead of 100mm).

Manic - the LSD and non LSD look identical from the outside (unless the identification tag is still on there). Also, if you take the back off an LSD all you can see are the normal gears - making some people think they've got an open diff. Easiest way to tell is to try and turn the output flanges in the opposite direction - should be very difficult by hand on a healthy LSD but almost no resistance on a non LSD. Also, if the back is still off look at the location of the diff mechanism (the four spider gears). If it's central between the output flanges its a non LSD, if it's off to the right side (looking at the back) and central with the diff input flange it's an LSD.

Hope that helps

Liam

EDIT: Replaced PCD with flange size - oops

[Edited on 12/12/05 by Liam]

To answer Chrisf - i can't see you having major problems. Like i said i think the flange spacing is the same between 7 and 7.5 diffs. Would be nice to get this confirmed though - anyone got a 7.5 diff and handy with measuring sticks??

If you were to import a 7" LSD later, therefore, it ought to go in no problem. You'll have to get 1/2" of shim/washer to bring the top mount in and your propshaft sliding joint will have to give up another 5-7mm. Even if one driveshaft does need to be 1/2" longer there probably is enough travel in your shaft (like I said they have lots of float), but at worst you'll need to import a shaft from a 7" with your 7" diff - no need to have one made.

Or you could just swap to a 7.5" LSD. But dont get a cossie one as it has the larger bolt pcd for the driveshafts so you'd need two cossie shafts too. If you get a 7.5" LSD make sure it's from a Granada Scorpio 24V Cosworth (pre bugeye facelift) and not a sierra.

Having said all that I'm not totally sure of the spec of these US Merkurs. Check your driveshaft flange diameter to be sure (I expect it's 100mm with 86mm bolt PCD). Ooooh and the murkur has drum brakes doesn't it??? In which case you will definately have to stick with your shafts - there is no bolt on shaft/drum brakes combination in the UK, so all our bolt on shafts have ends for disc brakes (different I believe).

Hmmmm hope that helps

Liam

EDIT: As above

[Edited on 12/12/05 by Liam]

Can someone clarify the deal with drive shaft lengths please. I thought all Sierras had un-equal length shafts or was it just a few?

Cheers

Rich

They all have unequal length shafts. Left/passenger (uk) is about 40mm shorter than the other one.

Yeh, thats what I thought, managed to confuse myself though Thanks for that

Rich

Merkur Lobro/shaft bolt pattern is 6 on 86 mm

Oooh er you've just made me realise i'm getting my terms a bit muddled!! It's the flange overall diameter that is 100/108mm, not the PCD of the bolt holes! PCD on normal 100mm flanges is indeed 86mm - so looks like the merkur stuff is compatible with UK stuff.

Liam