Newbie ordering kit starting parts collection for full MKIV IRS.

[Neuxstone]

Just heading into wild blue retirement yonder!!!
The wife loves the thought of keeping me busy so she GREEN LIGHTED the MKIV build late winter.
I did a search but got muddled up with generic information.
Question (first of many in the next few months)(followed by many more later I'm sure)
is...: I'm looking for a good used IRS. What one should I be looking for? Where should I be looking? And like everything else is going to be in this roadster; should I get a new/rebuilt.
Id like to stick with Ford products.
Does the T-bird or 95+ Mustang sport these? I've seen the Lincoln MKVIII suggested...help please if you have any info.
Thanks. Dave.

[Jacob McCrea]

1. Welcome and congratulations on the retirement.
2. A used Ford 8.8" IRS differential can be obtained from the following vehicles: 1989-1997 Ford Thunderbirds, 1989-1997 Mercury Cougars, 1989-1997 Lincoln Mark 8s, and 2003-2004 Cobras (and maybe 99-01 Cobras – I think those were IRS as well). If buying used be careful as some of the Thunderbirds and Cougars came with 7.5" differentials, not 8.8" differentials. The 8.8" will be cast into the aluminum cover, on the top of the passenger side if I recall.
3. The Thunderbirds and Cougars came with iron cases; the Mark 8s and Cobras came with aluminum cases. The aluminum cases are quite a bit lighter.
4. The Cobra unit came with a 31-spline limited slip unit (except maybe the 1999 unit - again I can't say with certainty). The FFR half-shafts are 28 spline. So, if you want to use the Cobra unit you need to either change to a 28-spline limited slip unit or change the inner half-shafts to 31-sline units. Some Google searching will pull up threads from either this forum or the other on how to do the latter.
5. It is very hard to find a differential from a Thunderbird, Cougar or Mark 8 with a factory limited slip unit. I think the Thunderbird "Super Coupes" and Cougar XR-7s came with them, but the vast majority of the used differentials out there will be an "open" differential. The factory may have built a few Mark 8s with a limited slip, but I have never seen one. The bottom line: if you want a limited slip differential, you will probably have to obtain the parts separately and "piece it together," or have someone build it up.
6. The Mark 8s can be nice pieces since (in my experience) they came with 3.27 gears more often than the Thunderbirds and Cougars. That saves you the trouble of obtaining new gears if you want that ratio, and the factory-set pinion depth will be just about right, if not perfect. The downside is that, as mentioned, they will probably not have a limited slip, and you should use a case spreader to change out the limited slip unit/set the backlash.
7. As for factory limited slip units, I believe they were discontinued as I have not seen them in the FRPP catalog for a while. Torsen, Detroit Locker and perhaps Auburn sell 28-spline limited slip units. I set up my IRS with a Torsen unit, but have never driven it (still building).
8. If you have trouble sourcing a differential locally, I have a few at my shop and would be happy to build one to your specs for a reasonable price. I also know a fellow who seems to have a bunch of these and can give him a call to see what he currently has.
9. Let me know if you want me to post a picture of the homemade case spreader and/or some other tools I have built for working on these things.
10. Again, welcome and congratulations on retirement. I hope this helps.

[Neuxstone]

Thanks for the thorough answer and your offer to help. I can already see this is going to be a fun build. I'll conduct any more questions through private message.
Actually I failed my first retirement and started a business building alloy gas tanks for antique motorcycles and the latest Cafe Racer craze. Legendary-Motorcycles.com. We've done contract builds on Vincent Egli tanks all the way up to modern Ducati race tanks. I have a whole shop full of metalworking equipment. I think I can handle a Cobra but I'll have to get a bigger English Wheel.
Thought I'd try my hand at an alloy body for my build. Because of the extra labor being exacted on it I will most likely have to rely on ready to bolt on parts.