by D. Brian Smith
(Taken from How to build Cobra kit cars + buying used by D. Brian Smith)
A crucial part of the Factory Five Cobra MK4 assembly is fitting the differential and rear suspension to the chassis. A solid axle or an optional three-link rear axle are available through Factory Five. Over the years, most Cobras have been assembled with the solid axle ear. However, for this particular build, we installed the new independent rear suspension from Tasca Ford in Massachusetts.
1. Rotate Differential into Place
The correct way to install the differential is to place the unit on your hydraulic floor jack vertically. Point the nose (driveshaft flange) upward and align the halfshaft holes front to back with the chassis. Slowly jack the pumpkin into the IRS cage of the frame, while someone else holds the differential steady to keep it from falling. Our floor jack did not have a high enough lift, so we improvised with a roll of duct tape. A block of wood works better.
With the pumpkin in the cage, correctly position it while it’s resting on the floor jack. Rotate the differential so that the holes for the halfshafts run longitudinally. Tilt it forward to get the center section correctly positioned for installation.
2. Install Rear Bolt to Hold Differential
A bolt holds the differential housing bracket on the subframe. Use a socket and ratchet to torque down the locknut. But first, just fingertighten the bolt so you can align the differential with the other fittings. Use a rear bolt through the rear cover to hold it in place. The center section weighs at least 50 or 60 pounds, so you may need to use a floor jack.
3. Install Center Section Bushings and Washers
Install and align all bushings and washers in preparation to install the bolts. Carefully lift the front of the IRS center section and slide in one washer and one bushing for each side of the differential’s front attachment ears. Be sure to have good support for the differential so you don’t pinch your fingers. Use a large Phillips-head screwdriver to line up the bolt holes. Place the bushing and washer on top of each attachment ear, line up the holes, and then remove the screwdriver to drop the bolts in the holes.
4. Setting the Torque
The center section of the independent rear suspension looks very clean. Torque the bolts on the front that travel through the bushings, and then turn your attention to the rear of the differential and torque that bolt. All four of the differential fasteners, the two through the bushings in the front and the rear fasteners, get torqued to 110 ft-lbs.
5. Install Lower Control Arms
Use a 15/16-inch combination wrench and a ratchet and 15/16-inch socket to handtighten the rod ends with jam nuts into the lower control arms. Handtighten and then back them off four turns. Then install the lower control arms to the chassis with the shock installation mount hanging below the arm. Use three shims on the front side of the front rod end for both the left and right arms. Also install all of the supplied shims on the other rod ends on either side.
We found this extremely difficult and so may you. It’s a bit easier to use a stool. Be as patiently precise as possible. When the car is finished, you need to have an alignment shop professionally install the appropriate number of shims and properly align the chassis.
With this accomplished, tighten all four bolts on each side, but don’t torque yet. You are going to remove the two rear bolts, install the right number of shims, and align later. You can see it’s also a good idea to prop up the lower arm with a stool or something to make your job easier.
6. Inspect Upper Control Arms and Rear Coil-Over Shock Assembly
The upper control arms, Koni coil-over shocks, roadster/coupe rear shock kit, snap-ring pliers, 3/4-inch wrench, 3/4-inch socket, ratchet, and ruler are all used to finish the IRS installation.
7. Install Upper Control Arms and Assemble Coil-Over Shocks
Place the lower shock eye in a vise and use a spring compressor to compress the spring. Then use the snap-ring pliers to install the snap-ring collar on the shocks. Very carefully follow the same correct procedure you used to assemble the front coil-over shocks.
8. Mount Rear Shocks on Suspension
Begin the installation process by mounting the coil-over to the chassis mount. Guide the bolt through the shock-mounting bracket and shock eye. Use two of the smaller-length spacers, one on either side of the shock eye. Align the lower shock eyes on the bracket of the lower control arms, guide the mount bolt through the eye, and use the large spacer. As with the front shocks, the shock body goes toward the top. Don’t torque the rear shock bolts yet; wait until the suspension is aligned.
9. Inspect CV Axles, Upper Control Arms, Hub Carriers and Hubs
How cool do the CV axles, upper control arms, hub carriers, and hubs look? Even better than their looks is how well they perform when installed in an FFR roadster IRS chassis.
10. Install Passenger-Side CV Axle
After you’ve placed towels on the arms to prevent scratching the CV axles or the lower control arms, carefully push the CV joints into the differential by hand and rest the CV axles on the lower control arms. The arms are in position if you can feel and see when they reach their appropriate position.
11. Install Hub on Spindle
Make sure that the hub is even with the spindle and press it into the spindle opening. Use your workbench vise to make sure that the hub is pushed down into the spindle evenly. Before putting the hubs through the spindle, they may require a small amount of fine and even sanding so they fit in the spindle holes. Because our CV axles were fully assembled, we didn’t have to put them together the way the FFR MK4 Complete Kit assembly manual details. But you may need to.
12. Inspect Suspension Components
Installing these components may be the most challenging step in building your IRS. The rear spindles/hubs, fasteners, upper control arms, and ears for upper control arms all have come together to create one unit. The disc-brake caliper mounting holes are on the back part of the spindles when you install them.
13. Bend Out Control Arm Lower Attachment Ears
If the attachment ears on the lower control arms are too narrow to accommodate the spindle sleeves, you can wrap a shop rag around the attachment ear to protect the black powdercoat. Use a hand vise and a pry bar to pry out the ears before negotiating the spindle sleeves into their new home. Prying against the hand vise enables you to pry out the entire attachment ear in an even fashion. Slide the hub assembly and spindle on the outer CV joint.
14. Bend Upper Control Arm Attachment Ear
The attachment ear for the upper control arm to the top spindle sleeve also needs a bit of fine-tuning. Use the workbench and a trusty shop rag for protecting the powdercoat to offer some stationary and stout leverage. You now know why many mechanics have Popeye forearms.
15. Tighten Hub Bolt
Tighten the three fasteners for each rear spindle and the 36-mm hub nuts, but don’t torque them yet; the independent rear suspension is aligned later.