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When selecting a donor vehicle, there are several important things to look for to avoid problems. Ford
used the same IRS set-up for a few different cars. Thunderbirds between 1989 and 1997, Lincoln Mark
VIII's between 1992 and 1999, and Mercury Cougar XR7's between 1989 and 1992. Only some of the
cars have the correct parts that are needed. The points to check are as follows:
1. Check the center section. What you need is an 8.8 inch unit. It will have 8.8 cast into the top of the
differential and is noticeably larger than the 7.5 inch center that was also used. Lincoln Mark VIII
came with aluminum center sections that are 15lbs lighter than the iron one.
2. The Thunderbirds were available with and without rear disc brakes. We recommend one with the
discs (free upgrade from a drum brake solid axle).
• When you find a rear you like, ask for the whole rear clip from the car. The whole thing unbolts with
four bolts and they usually cut the driveshaft. This way you will be sure to get everything that you
need.
• The rear ends we have found work the best are from Thunderbird Supercoupes. We have found them
for between $300-$350. Lincoln Mark VIII's go for around $600.
Limited slip (Posi-traction/Traction-Lok) and Open Differentials
• Early Mark VIII center sections do not have a limited slip differential.
• The Thunderbird Supercoupe does have a limited slip differential.
• The IRS is an 8.8 center section and it can use the same gears that the solid axle 8.8 from the Mustang
uses but, it can not use the same differentials. The IRS differential has a step cut in the side gears to
hold the CV joints in the center section. Without this step there is nothing holding the axles from
coming out.
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508-291-3443
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