IRS Install: Test fit and bracket modification

The IRS fits in the foxbody Mustang, but modifications need to be made to bolt it in.  Once everything is removed test fit the cradle to make sure it goes in right and nothing is bent or mis-aligned.  Now’s the time to find out and make modifications if so.  Luckily mine lined right up with the help of some rubber hammers and a little persuasion.  The first thing to tackle is the brackets that bolt to the rear sub-frame, new holes need to be marked when the cradle is in.  Many IRS swaps that are done, people have drilled through the frame and just used a through bolt and a nut & washer on the other side and called it a day.  Realspeed had another idea, a slightly more time consuming process, but the final results would be more secure and look factory.

There are 4 bolt locations on the bracket, 2 from the side and 2 from the bottom.  Only one lines up to the frame.  The picture below diagrams this and the only hole that is usable is the quad shock hole, so a quad shock bolt can be used to hold the bracket here.  For the other 3 holes, the plan is to weld a nut to the inside of the frame, so the bolts can screw into them.


Hole #2 is cut out with a hole saw bit, large enough to fit a nut and washer behind it to be tack welded.  The factory hole is cut out as well to make enough room to push a plate through…see pictures #2 & #3 below.


Rob is using a 1/2″ bolt for hole #2, so he welded a washer to a nut.  You need two of these total, one for each side.


Homemade nuts

He then made this welded plate/nut for holes #3 and #4, in which he is using 7/16″ bolts.  You need to make two these, one for each side.


With the hole #2 large enough, Rob passes through the plate and sets it to line up with holes #3 & #4 at the bottom of the sub-frame, then tacks it in.


Tack weld the washer/nut in place, make sure it is lined up correctly.

Custom washer/nut


Then add a little steal to the exposed hole, grind, sand and paint.


And the IRS cradle bracket is bolted in, perfect fit.  Looks factory.

IRS bracket

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