Factory Five Racing Mk3 Roadster Assembly Manual page 250

• There are only three differentials that will work in either the Mark VIII or T-bird center section, the
stock Supercoupe traction-Lok, an Auburn Gear differential and a Torsen differential. Auburn offers
two units that will work. The high performance unit (part # 542079) for standard OE replacement
(~$250), and the pro series unit (part # 542080) for better traction and faster engagement (~$350).
The Torsen, sold by Ford Motorsport is the best differential around and is used by many performance
car companies (Porsche) as standard equipment. It is all gears inside and never wears out. It isn't
designed for a life of drag racing though, it's designed for roadracing. The Ford Motorsport part
number is M-4204-T28 (~$500).
IX. Modification of IRS parts
Note:
• Skip this section if using the IRS Completion Package and go to the Installation section
• Do not throw away any parts from the IRS until the IRS is installed.
• Spindles
Note: The Thunderbird uses a 5 lug x 4.25" Lug pattern. This is not very helpful since the Mustang uses
either a 4 lug x 4.25" pattern or a 5 lug x 4.5" pattern. Unless you want a different lug pattern on
your rear, the hubs will have to be drilled to the 5 lug x 4.5" pattern. We would recommend that a
machine shop do this, as the measurements need to be precise otherwise your wheels will not fit.
We had a shop do both hubs for $100. The machine shop will press the hubs out of the bearings
and press the wheel studs out of the hubs. We have included new ½"studs so that the front and
rear lug nuts are the same and are good and strong. The Thunderbird used metric studs and lugs
which should not be used.
When you get the spindles back, check the hubs for slop in the bearings. There should be none. If
there is, have the shop replace the bearings. Check the new bolt pattern in a wheel to make sure
that it will go on.
Wheel studs in new location.
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