IRS Build-up How-To

The rear suspension system has the following characteristics: Permanent mould cast aluminum rear wheel knuckle: Replacing the Subframe isolator bushings on high milage cars is a good idea. Front part # E9SZ5D006B (2 required) Rear part # E9SZ5K617A (2 required).

Press the old isolators out. Inspect and clean the opening. Grease new bushings and press in.

The IRS differential bushings should be inspected and replaced. Now available are Polyurethane Differential Bushings.
Now available is the MN12 Performance Pinion Brace.

The Pinion Brace adds addition support for the Differential and subframe.

Subframe removal can easily be done using a transmission/differential jack.

The IRS has several weak points that can be strengthend and the first is the halfshafts.

The rear wheel drive halfshaft system is assembled and installed as follows:

Stock interconnecting shafts are 1 1/16" between the inner and outer CV joints. The stock interconnecting shaft can break under heavy acceleration like drag racing while using slicks. A company Raxles has developed replacement halfshaft that employs "Ford of Germany" inner and outer CV joints designed for use on the German Autobahn while using a custom 1.5" interconnecting shaft made from hardned steel for added strength.

Next area that needs strengthing is the IRS subframe

The IRS subframe shown here removed from the vehicle it is made of stamped 1/8" steel welded together. The areas that need strengthing are around the control arm boxes to reduce flexing. Ford left areas around the control arm boxes weak and additional welding is needed along with reinforcing the boxes with square tubing. The picture below is in progress and not completed. Adding 1/2"-3/4" square tubing to the bottom of the control arm boxes adds additional strength to the entire subframe structure. I picked the tubing up at a iron rail fab shop for $20 bucks. I also used the same tubing for my jacking rails. The area of the control arm box that really takes alot of the stress are the two corners on each box.

To see additional pictures of the lower control arm boxing click link.

The rear lower control arm boxes have been boxed and the front are getting ready to be fitted with a section of square tubing.

Front lower control arm box is bigger than the rear lower conrol arm box and needs the support bar welded in at the line. Check for complete clearance before welding. Alignment of the rear depends on it! Test fit the lower control arm with the support tack welded before the final weld is done.

Rear lower control arm needs alot of welding to strengthen it. Any gap or seam unwelded gets a weld added.

Final area that needed attention was the upper control arms which are made from 1/8" stamped steel. Adding additional support will help keep flex down. Note: Be sure you have the tubing back far enough so it does not interfear with the control arm motion. Replacement Upper/Lower Control Arm bushings are available through Ford, AC Delco and Moog. Lower control arm bushing (front side) 45g11057 (AC Delco) or F1SZ-5K790-B (Ford) and Lower Control Arm bushing (rear side) k8658 or F1SZ-5K790-A (Ford). Check toe compensator link for a loose ball and socket fit. If worn replace the toe compensator link which is available from Ford 5A972 or Moog K8594.

To see additional pictures of the upper control arm boxing click link.

Box the outside of the upper control arm to allow full clearance of the knuckle bushing when installed. Test fit before making the final weld.

On the inside of the upper control arm you will have to recess the support inside. If you don't the half shafts will touch the support under heavy acceleration. I also added and additional support on the lower side of the bushing to provide less flex. New upper control arm bushings are available from Ford and Moog (k8562)


Rear of car supported by 6 ton jack stands.

Unbolt rear suspension parts from subframe. 36mm socket is needed to remove the halfshaft nut from knuckle. 3 arm 7" pulley puller was used to remove the halfshaft from the knuckle. Then remove the exhaust cat-back.

Suspend the halfshaft until the exhaust and lower control are are removed.

The differential and halfshafts weigh around 150 pounds so two people maybe needed to lower it. The subframe only weighs around 50 pounds. Disconnect the Emergency brake cables and the 4 subframe support bolts then drop the subframe out.

3.31 differential with limited slip and halfshafts removed. You do not need to remove the rear cover to remove the halfshafts.

Pictures shows how the halfshafts fit inside the differential.

New "stronger" subframe installed. Polyurethane Spring Isolators are now available to replace the OEM rubber.

Now available are Polyurethane Shock Mount Bushing Kits.

Suspension reinstalled with HD halfshafts. Now available are Polyurethane Rear Knuckle Bushing.

Once the

Since I had been using a Addco 1 1/8" rear sway bar I decided to modify the endlinks to provide better life as the Addco endlinks that come with the sway bar tended to shift under cornering. The OEM uses a bayonett style bushing which is welded onto the Addco sway bar. Now available are Polyurethane Sway Bar Bushings and End Link Kits for Stock sway bars ONLY.

Using the origional endlink bracket on both ends. Positioned the brackets using the OEM sway bar so they were the correct length apart. I used the OEM nut to hold the bracket in place. Take the control arm bushing out as the heat from the welding will distort it. Add a couple weld beads and your done. I used stainless steel wire throughout my entire IRS build-up so future rust will be kept to a minimum.

I reused some of the polurethane endlink bushings.

MN12 Performnce Inc.