Mark VIII 8.8 IRS

[Kpt112]

I have located a Mark VIII IRS from a 1993 Lincoln. Can anyone share any gotchas with this particular IRS. I have verified it has a ratio of 3.8 and an 8.8 gear ring. I am assuming it has 28 splines. The guy is selling me the rear along with the entire carriage assembly. It has 127k miles on it. Since this is the first time I am doing this anyone that can share some experiences will help me avoid mistakes and potential problems. I am going to take it to a transmission shop for the once over. If it looks good is there anything you would absolutely recommend swapping out, (bearing, bushing etc.)

Thanks as always
Kevin

[Bren]

I think the consensus on the forum is to completely rebuild the internals. Others can chime in with more experience than I have, but depending upon the condition of the diff and what kind of motor you'll attach to it, I think you'll be better safe than sorry to rebuild the internals. Assuming you're getting the complete kit, everything else on this car will be brand new. Why would you put in a diff that's 20+ years old?

[oldguy668]

The MK VIII did not have a limited slip differential. Lincoln relied on using the anti-lock brakes to control wheel spin. You will probably have to budget a new diff unit. The good news is it's an aluminum case.

[bansheekev]

Just to level set, I got my aluminum pumpkin and outer IRS uprights/hubs from the local pick n pull off of a 1997 Lincoln MKVIII with 110k miles for $100 for everything. The differential was a 2.73 open diff. I would encourage you to insist on seeing the metal tag on the differential to validate what is in there. You can google what the code stamped on the tag means. I didn't pull the entire IRS unit as I wasn't using axles or brakes from the Lincoln. I gutted the pumpkin and purchased a new Eaton Truetrac differential, new Ford Racing ring and pinion, new bearings/seals, and had a local guy build it. Entire setup parts and labor cost $950.

[stroked 6t5]

Hi all,
The MK8 is the diff of choice simply due to the fact it's aluminum. The T-birds are all cast. If it were me buying it, I would take the case and leave the rest. I would then take it to a reputable shop and have it completely rebuilt with posi and the ratio of your choice. Just my $.02.
Regards,
Andy

[CraigS]

I wouldn't get hung up on the aluminum case. I'd rather get a diff w/ a good gear ratio and/or a limited slip even if it's iron. The Ford lim slip can be rebuilt w/ a $50 kit so much cheaper than buying a trutrac. Not needing new gears would be a huge advantage $ also.

[Jacob McCrea]

If it looks good is there anything you would absolutely recommend swapping out, (bearing, bushing etc.)

Thanks as always
Kevin

I agree with Bren's comments. When you consider what a rebuild kit costs and what a hassle it would be to replace, say, a failed pinion bearing or seal, it makes good sense to install all new bearings and a pinion seal while it's relatively easy. The same is true for the CV joint bearings and accompanying seals. ARP, Summit and Trick Flow make high strength studs for the carrier bearing caps; I would not miss a chance to install them, although I've never heard of the stock bolts failing.

[Jacob McCrea]

Here is an example of why it is worth it to tear these things down and at least inspect everything. This picture is the driver's side carrier bearing race from an iron 8.8 IRS open differential with around 130k miles. You can see that the race is starting to get rough. The other bearings and races were fine, the pattern looked good and the pinion preload was perfect for used bearings, but the backlash was .002 beyond spec, which I'd guess is just a product of ordinary wear.

39181

I pulled it apart last night and am going to completely rebuild it, paint it, add a Truetrac limited slip and post it for sale here for what I hope is a reasonable price.