TCCoA Forums banner
Not open for further replies.
1 - 1 of 2 Posts

498 Posts
IRS Build-up How-To

The rear suspension system has the following characteristics:

  • It is fully independent and is mounted on a rubber isolated

  • Large rear suspension arm and bushings (H-arm configuration)
    are used to oppose torque reaction on braking and acceleration.

  • A rear suspension compensator link controls toe-out during

  • The stamped rear suspension arm and bushing provides camber

  • The variable rate rear spring is mounted between the rear
    suspension arm and bushing and the body.

  • A rear suspension jounce bumper is located inside the rear
    shock absorber.

  • A tubular rear stabilizer bar is standard on all models.
Permanent mould cast aluminum rear wheel knuckle:

  • Carries the rear suspension loads.

  • Connects the upper rear suspension arm and bushing assembly
    and lower rear suspension arm and bushing assembly.

  • Houses the rear wheel bearing.

  • Provides mounts for the brakes.
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

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:

  • The system employs constant velocity (CV) joints at both
    its inboard (differential) and outboard (wheel) ends for vehicle operating

  • The CV joints are connected by an interconnecting shaft which
    is splined at both ends and retained in the inboard and outboard CV joints
    by driveshaft bearing retainer circlips.

  • The inboard cv joint stub shaft pilot bearing housing is
    splined and held in the differential side gear by a driveshaft bearing
    retainer circlip.

  • The outboard CV joint/interconnecting shaft is pressed into
    the rear hub and secured with a rear axle wheel hub retainer.

  • The CV joints are lube-for-life with a special CV joint grease
    and require no periodic lubrication.

  • The rear axle shaft outer boot / rear axle shaft inner boot
    , however, should be periodically inspected and replaced immediately when
    damage or grease leakage is evident.

  • Continued operation would result in CV joint wear and noise
    due to contamination or loss of the CV joint grease.

  • Halfshaft removal from the differential is accomplished by
    applying a load to the back face of the inboard CV joint assembly to overcome
    the driveshaft bearing retainer circlip.

  • The outboard joint end must be pressed from the rear hub.

  • The inboard tripod CV joints can be disassembled and serviced.

  • Other than the rear axle cv joint boot , the outboard CV
    joint is serviced only as an assembly with the outboard CV joint/interconnecting
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.
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.​
1 - 1 of 2 Posts
Not open for further replies.