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Old 02-12-2009, 07:37 AM   #1
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THUNDERbird and Cobra INDEPENDENT REAR SUSPENSION question

alright, going to get started on my IRS swap here in a little while. got a problem though...and a possible solution I think.

ok, bought a 99 cobra IRS setup for SUPER cheap. reason being is the center housing was cracked from a hard launch (tisk tisk). anyway, my dad has a 94 tbird supercoupe with the iron center section. I figure for the little extra weight from the iron versus aluminum, I'll still win out since its free and iron is usually a little stronger from what I've seen with rearends.

question #1. going to pull the tbird center chunk this weekend. how are the axles held in?? do they have c-clips inside the chunk like regular 8.8"??

question #2. anyone know if the different bearings are the same size between the two units as I was going to swap the diff from the busted cobra unit into the tbird unit.

question #3. anything I'm not thinking of??

I could wait for a mark VIII aluminum rearend to pop up in the local u-pull but hey, this one is free and I dont really care about adding 40lb to the weight of the car personally.
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Old 02-12-2009, 01:20 PM   #2
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Are you saying you want to put the Cobra carrier in the iron TBird housing?
Why, isn't the TBird unit you have a traction-lok as is the Cobra? If so, they both have the same carrier.
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Old 02-13-2009, 01:57 PM   #3
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Quote:
Originally Posted by kennethl6 View Post
Are you saying you want to put the Cobra carrier in the iron TBird housing?
Why, isn't the TBird unit you have a traction-lok as is the Cobra? If so, they both have the same carrier.
tbird is 28 spline and the cobra is 31 spline. need to use the cobra carrier so I can use the cobra cv joints...which are the correct length for the irs unit
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Old 02-13-2009, 03:06 PM   #4
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Originally Posted by stackz88 View Post
tbird is 28 spline and the cobra is 31 spline. need to use the cobra carrier so I can use the cobra cv joints...which are the correct length for the irs unit
I think the 99 IRS is a 28 spline - 01 and 03/04s are 31 spline.
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Old 02-13-2009, 03:25 PM   #5
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Quote:
Originally Posted by qtrracer View Post
I think the 99 IRS is a 28 spline - 01 and 03/04s are 31 spline.
03/04 IRS is 31 spline, 99-01 is 28 spline. Both the Tbird and Cobra center section are 8.8 (as long as the T-bird diff is from a V8 car) so all parts interchange. IIRC the axles are held in with a retainer clip that pops out of the half-shaft into a groove in the diff, you just compress it and pull the shaft out.

Also the T-bird and Mark VIII diff are not trac-loc diffs. Don't ask me why Ford would put a 290 hp V8 in the Mark VIII with an open diff but they sure did. Use the Cobra trac-loc.
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Old 02-13-2009, 05:08 PM   #6
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Quote:
Originally Posted by PritchSVT97 View Post
03/04 IRS is 31 spline, 99-01 is 28 spline.
Uh - I have an 01 cobra - 31 spline.
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Old 02-13-2009, 05:22 PM   #7
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Uh - I have an 01 cobra - 31 spline.
stock?? hell, makes me want to go back there tonight in the garage in the cold and pop the cover and one of the cv joints and count splines.

well, regardless I guess the main question I was trying to ask is will the cobra diff fit in the tbird housing??

are the diff bearings the same size?? I've heard "rumors" of larger and smaller bearing housings, etc.

never actually SEEN it per say but I figure if there was ever a time I'd see it, it would be this time lol.

oh and there were several 3.8 supercharged cars that had lockers in the back. It was a package option for both the SC and the XR7. we've had 6 cars come through my dad's salvage yard and 2 had lockers from the factory in them, even had the tag on the rear as well as door tag ID. no clue on this one though, it was hit hard in the left side and is on the ground without wheels and the forklift is having the carb rebuilt right now lol

Last edited by stackz88; 02-13-2009 at 05:24 PM.
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Old 02-20-2009, 10:00 AM   #8
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The '99 Cobra carrier diff is a tbird carrier. All the bearings are the same in a 31 spline. You will need to reshim the sideplay and possibly the pinion when you swap housings. A Lincoln MarkVIII has an identical aluminum housing. There is a difference in covers. I haven't seen housings break, but covers break often.
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Old 02-23-2009, 07:58 AM   #9
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I haven't seen housings break, but covers break often.
ok, I'll post a pic so you can see a broken housing
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Old 02-23-2009, 06:40 PM   #10
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Quote:
Originally Posted by qtrracer View Post
Uh - I have an 01 cobra - 31 spline.
I may be wrong, but 03-04 Cobra 31 spline inner and outer, 00-01 31 spline inner/28 spline outer, and 99 28 spline inner and outer.
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Old 02-24-2009, 08:54 AM   #11
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I may be wrong, but 03-04 Cobra 31 spline inner and outer, 00-01 31 spline inner/28 spline outer, and 99 28 spline inner and outer.
really? well that would make the swap even easier lol. I'm not putting out ground pounding #'s anyway with my svo, just want better handling in the hard turns so 28spline or 31 spline really honestly doesnt matter that much to me.
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Old 02-24-2009, 11:58 PM   #12
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So will a Thunderbird IRS work on a Mustang the same as a Cobra IRS? I am not running big HP numbers either, so the 28 spline doesn't matter to me. An I know I could find a T-Bird rear pretty easy.

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Old 02-25-2009, 07:27 AM   #13
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only the center chunk can be used as far as I know. the MN12 chassis of the tbird uses different attachment points for the subframe assembly of the IRS as far as I know.
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