LSD unit swap between 7.5" differential units
Discussion
Dear all,
I have a 1990 S3C and swapped in a different 7.5" Ford Sierra diff with a viscous race LSD a few years ago. The diff came out of a RS500 that someone crashed at Truxton. I also bought some other parts at the time.
I think the original is 3.64:1 but the new one is 3.14:1 which seemed a good idea at the time. I still have the original TVR diff on the shelf. I used to run her supercharged and then the torque was enough for the low gearing, without the additional respiration help, not so much.
However, I would like to re-install the original diff to rise the gearing as she is running outside the power band most of the time. Today she runs a different cam, ported plenum/heads and some other mods which makes her a bit slow under 2.5-3k RPM. But better behaved above https://www.pistonheads.com/gassing/imgs/2.gif
Hence I wonder if the LSD unit from the 7.5”/3.14 diff can be direct swapped into the original 7,5”/3.64 diff housing. Are the internals always the same in the Ford Sierra 7.5” diffs?
If so, do I need to take the entire diff apart to swap over the LSD unit? Link to a good wiki would be much appreciated.
Kind regards
//Rob
I have a 1990 S3C and swapped in a different 7.5" Ford Sierra diff with a viscous race LSD a few years ago. The diff came out of a RS500 that someone crashed at Truxton. I also bought some other parts at the time.
I think the original is 3.64:1 but the new one is 3.14:1 which seemed a good idea at the time. I still have the original TVR diff on the shelf. I used to run her supercharged and then the torque was enough for the low gearing, without the additional respiration help, not so much.
However, I would like to re-install the original diff to rise the gearing as she is running outside the power band most of the time. Today she runs a different cam, ported plenum/heads and some other mods which makes her a bit slow under 2.5-3k RPM. But better behaved above https://www.pistonheads.com/gassing/imgs/2.gif
Hence I wonder if the LSD unit from the 7.5”/3.14 diff can be direct swapped into the original 7,5”/3.64 diff housing. Are the internals always the same in the Ford Sierra 7.5” diffs?
If so, do I need to take the entire diff apart to swap over the LSD unit? Link to a good wiki would be much appreciated.
Kind regards
//Rob
What I think you need to do is swap just the crown wheel and pinion from the std diff into your LSD. However, I'm not sure that it's entirely a diy job as the crown wheel/pinion interface may need to be setup carefully.
If the LSD is good, just adjusting its gearing would give you the best of both worlds.
N
If the LSD is good, just adjusting its gearing would give you the best of both worlds.
N
h271mbk said:
What I think you need to do is swap just the crown wheel and pinion from the std diff into your LSD. However, I'm not sure that it's entirely a diy job as the crown wheel/pinion interface may need to be setup carefully.
If the LSD is good, just adjusting its gearing would give you the best of both worlds.
N
This is the way to goIf the LSD is good, just adjusting its gearing would give you the best of both worlds.
N
Hi,
Thank you for the replies.
Is there any difference in moving the LSD unit from the 3.14 diff to the original 3.64 diff compared to moving the pinion and crown wheel from the 3.64 diff to the 3.14 LSD diff?
The reason for asking is that my original 3.64 diff (no LSD) has only covered 5000 miles from new but I have no info on the 3.14 race diff.
Kind regards
//Rob
Thank you for the replies.
Is there any difference in moving the LSD unit from the 3.14 diff to the original 3.64 diff compared to moving the pinion and crown wheel from the 3.64 diff to the 3.14 LSD diff?
The reason for asking is that my original 3.64 diff (no LSD) has only covered 5000 miles from new but I have no info on the 3.14 race diff.
Kind regards
//Rob
The casings for etc for both diff should be the same. The only difference between the two diffs will be the LSD unit and the output shafts (LSD diffs have a different sized spline).
However, diff internals need to be set up to within fine tolerances so you can't just swap things in and out.
Why don't you get your LSD refurbished with 3.64 crown and pinion; that way you'll have a reliable well setup diff that should last for years.
N
However, diff internals need to be set up to within fine tolerances so you can't just swap things in and out.
Why don't you get your LSD refurbished with 3.64 crown and pinion; that way you'll have a reliable well setup diff that should last for years.
N
All Sierra Cosworth diffs were 108mm flanges, so if this came off a Cossie and it's 100mm, something's different already. Incidentally, the standard Sierra Cosworth diff is LSD, and 3.64. The downside is they're 108mm flanges, so I had to have custom driveshafts made for mine as I wanted to keep the 3.64 ratio.
If it's LSD and 100mm, it's probably a Granada one, or a Sierra XR4x4.
I know the output flanges aren't interchangeable between LSD and non-LSD, but if you've got an LSD diff with 100mm flanges then there's a chance you could use those in a Cosworth diff. Would have saved me a lot of grief if I could have found some at the time!
If it's LSD and 100mm, it's probably a Granada one, or a Sierra XR4x4.
I know the output flanges aren't interchangeable between LSD and non-LSD, but if you've got an LSD diff with 100mm flanges then there's a chance you could use those in a Cosworth diff. Would have saved me a lot of grief if I could have found some at the time!
Hi,
The 3.14 came off of a crashed RS500 as far as I know. I have already changed the flages to 100's as it is already mounted on the S3C. The original 3.64 diff still has the original 100 flanges, so both should be fine. The original 108 from the 3.14 flanges is on the shelf.
The reason to want to go back to the original diff is that it is matching numbers for the car and it is very low milage.
Kind regards
//Rob
The 3.14 came off of a crashed RS500 as far as I know. I have already changed the flages to 100's as it is already mounted on the S3C. The original 3.64 diff still has the original 100 flanges, so both should be fine. The original 108 from the 3.14 flanges is on the shelf.
The reason to want to go back to the original diff is that it is matching numbers for the car and it is very low milage.
Kind regards
//Rob
If I recall correctly the 100’s flanges just went straight in as a direct replacement for the 108's.
However, they do look a little different as the TVR S3C original 3.64 diff with original 100’s flanges are flat but the 3.14 108’s and the 3.14 100’s do have an outer ring that rises above the drive shaft mounting surface so the drive shaft kind of sits in that ring.
My guess is that there were many versions of these bits back when. I bought the LSD unit back in the late 90’s and the guy I bought it from also supplied the 100 flanges. I have no idea where they came from. The diff as stated above is supposed to come from a RS500, but I had no way to verify that. I did also get the intercooler for my supercharger and some other bits and they were from a RS500 unit at least  .
But from the above it should be possible to take the LSD unit out from the "RS500" 3.14 diff and move to the 3.64 (TVR S3C) diff while keeping the internals of the “new” 3.64 unit, if done by a person who knows what they are doing (ie not me :-) )? The internals are the same and an LSD unit will fit in to the non LSD crown wheels?? Is that a correct summary
Kind regards
//Rob
However, they do look a little different as the TVR S3C original 3.64 diff with original 100’s flanges are flat but the 3.14 108’s and the 3.14 100’s do have an outer ring that rises above the drive shaft mounting surface so the drive shaft kind of sits in that ring.
My guess is that there were many versions of these bits back when. I bought the LSD unit back in the late 90’s and the guy I bought it from also supplied the 100 flanges. I have no idea where they came from. The diff as stated above is supposed to come from a RS500, but I had no way to verify that. I did also get the intercooler for my supercharger and some other bits and they were from a RS500 unit at least  .
But from the above it should be possible to take the LSD unit out from the "RS500" 3.14 diff and move to the 3.64 (TVR S3C) diff while keeping the internals of the “new” 3.64 unit, if done by a person who knows what they are doing (ie not me :-) )? The internals are the same and an LSD unit will fit in to the non LSD crown wheels?? Is that a correct summary
Kind regards
//Rob
So you plan to have the LSD unit removed from the current different and installed in the original using the originals crown and pinion. The current flanges can then be used as they will fit the LSD unit.
Should be a fairly straightforward swap for a specialist. Specialist could also check the torque of the LSD unit to make sure it's still OK.
N
Should be a fairly straightforward swap for a specialist. Specialist could also check the torque of the LSD unit to make sure it's still OK.
N
Hi,
I took down the installed supposed 3.14:1 diff yesterday and it turned out to be a 3.64:1, just as the original TVR diff, but with the LSD unit. It must have had the gears changed over the years.
I also found a brilliand Excel calculation sheet here and it confirmes my speed/gearing. -Should have done that in the first Place...
http://www.super7thheaven.co.uk/components/sierra_...
Anyway, that means that the gearing from the factory is to low for me so I will need to move to 4.09:1 or 4.27:1 gears to get some revs on the engine. so I will start looking for gears to swap into either of my 3.64:1 differentials.
Then I will leave all stuff to a specialist (I have found one) to put together one good high gearing diff for my car.
Regards
//Rob
I took down the installed supposed 3.14:1 diff yesterday and it turned out to be a 3.64:1, just as the original TVR diff, but with the LSD unit. It must have had the gears changed over the years.
I also found a brilliand Excel calculation sheet here and it confirmes my speed/gearing. -Should have done that in the first Place...
http://www.super7thheaven.co.uk/components/sierra_...
Anyway, that means that the gearing from the factory is to low for me so I will need to move to 4.09:1 or 4.27:1 gears to get some revs on the engine. so I will start looking for gears to swap into either of my 3.64:1 differentials.
Then I will leave all stuff to a specialist (I have found one) to put together one good high gearing diff for my car.
Regards
//Rob
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