The Corral

Cobra IRS Gear Swap

Written by John Khoury Tuesday, 15 February 2000 18:00

Drivetrain

Part 1 of 2 - IRS Disassembly

Randy Haywood of True Blue Performance did the majority of the work for me and I am much obliged to him, Mark and Joe Stewart for all their help.

Questions? Email me.  

NOTE: The information presented herein is for educational purposes only. There are no guarantees of any kind including the accuracy or safety of procedure shown. All risk is with the user. Always use the correct safety equipment when performing any type of auto modifications.


10 steps to disassembling the 1999 Cobra Independent Rear Suspension (IRS)
    Step 1: Remove the catback mufflers and pipes. Four or five hangers are on each side. The picture at left shows why the 99 Cobra stock muffler (pre-fix) corks up so much power.  You can see that it is overheating due to the internal restrictions.

 
    Step 2: Disconnect the emergency brake cables and pull them completely out of the way. The cables are difficult to see in this image, but it's simple to find once the car is on the lift.

 
caliperoff.jpg (50454 bytes)     Step 3: Remove the rear calipers (Two bolts on backside of each caliper) from their mounting and cable tie the caliper to a structural member so the brake line is not damaged.

 
rotorout.jpg (51688 bytes)     Steps 4 & 5: Remove the rear brake rotors. Disconnect tie-rod.  The tie-rod will require some hammering on the suspension piece surrounding it to pop out.

 
tierodout.jpg (54429 bytes)     Step 5: Disconnect the lower control arm bolts (A-arm) from the spindle. You will not need to remove the upper control arm bolts. If you do remove or loosen upper control arm bolts, you'll most likely need to get the car re-aligned.

 
airhammeraxle.jpg (62417 bytes)     Step 6: Remove axle half-shaft. Usually, you need to hammer the axle to get it to pop out of the spindle. Randy Haywood came up with a less brutal method. He ground an air hammer bit down to a small tip, then used the vibrations from the air hammer to nudge the axle out of the spindle. There's a dimple in the center of the axle that the tip of the bit fits into perfectly. Almost purpose made!

 
pullaxle.jpg (62429 bytes)     Step 7: Remove the axle half-shaft from the differential. There should be enough wiggle room to pull the lower spindle away from the axle. Once the axle is out of the spindle, the half-shaft simply pops out of the differential with a good pull. No tools needed. Make sure you don't damage the dust covers on either end. - The axle won't have enough room to totally come out of the IRS due to interference by the shock absorber. Just leave it hanging.

 
axleout.jpg (45967 bytes)     Step 8: Unbolt the ABS sensors hanging on each side of the differential. It may be tough to see the sensor in this picture, but it is just left of the hole where the half-shaft used to be.

 
removedriveshaft.jpg (51200 bytes)     Step 9: Unbolt and remove the driveshaft. Note: make sure to check the indexing of the drive shaft with the pinion flange before the driveshaft is removed! When reinstalling, you will need to match the marks back up. The Independent Rear Suspension on the Cobras are notorious for going out of balance so be certain to install/reinstall the driveshaft in the same position! You can leave the front portion of the driveshaft connected to the transmission, but it may be in the way. Watch out for fluid leaking from transmission if drive shaft is pulled out.

 
pumpkinfrombottom.jpg (67004 bytes)     Step 10: Unbolt the four bolts holding the differential to sub-frame. Here is an underside shot of the differential. Unbolt the four bolts for the differential carrier. Two on the bottom, and two on the back of the differential. Note: It helps to keep one of the bolts in the rear differential to hold it up until you're ready to drop the whole unit. The front portion of the differential will lay perfectly on the cross member until you're ready to remove it.

 
unboltpumprear.jpg (56148 bytes)     Front bolts in place, rear bolts being loosened.

 
restoncrossmem.jpg (34206 bytes)     Front of the differential (driveshaft connection point) is resting on the sub-frame cross member until the rear bolts are ready to come out.  It sits perfectly in the cross member.

 
holdpumpkin.jpg (50829 bytes)     The differential is all aluminum so it is fairly light. You won't need much muscle to get it up or down. Here it is being tilted to lower it away from the sub-frame.

 
droppumpkin.jpg (48832 bytes)     Having two sets of arms helps.

 
vacant.jpg (53632 bytes)     IRS sans differential!

Click to see a full-size version of each image.
    Step 1: After removing the differential from the IRS sub-assembly, you'll need to find an empty five gallon bucket to drain the differential fluid. The five gallon bucket also comes in very handy to stabilize the differential while working on it.

 
drain2.jpg (46896 bytes)     Step 2: Drain differential fluid. The fluid is foul smelling so work in a well ventilated area.

 
bucket.jpg (82991 bytes)     Step 3: Remove the differential cover and thoroughly scrape the housing clean so that it is free from any of the old gasket material in the receiver grooves of the cover and differential. Thankfully, you will not need to buy a gasket to reassemble the differential, high temperature gasket sealer is all that is necessary.  The 31 spline differential on IRS equipped Cobras does not use C-clips so disassembly is slightly easier than with solid axle 8.8" rears.

 
caps.jpg (54646 bytes)     Step 4: Mark the right and left caps while they are still installed in the differential. Remove the main caps. The caps are beefy and made out of aluminum just like the differential housing.  NOTE: When replacing the caps, put them back on the same side as you found them and make sure the arrow on each cap also faces OUT. See photo. If you look closely, you can see the arrow midway between the top and bottom cap bolt.

 
removediff.jpg (67529 bytes)     Step 5: Remove the traction-lok and ring gear unit.  In order to make this process easier, use the appropriate size closed-end wrench on one of the ring gear bolts to keep the housing from turning. Then using a wrench to spin the pinion nut will push the traction-lok unit out of the differential housing.

 
removeringgear.jpg (82090 bytes)     Step 6: Once the traction-lok and ring gear are out of the differential, remove the bolts holding the old ring gear to the differential. You will need an impact wrench to do this. In this case, we are installing a set of 4.10:1 gears which you can see sitting in the box on the left.

Once the old ring gear is unbolted, reverse the procedure to install the 4.10 ring gear. Make sure to tighten the bolts in a cris-cross pattern EVENLY, using the factory recommended torque specs.

 
pinion.jpg (52085 bytes)     Step 7: Remove the pinion gear by unbolting the pinion gear nut. In this photo, a lever was temporarily bolted to driveshaft flange using the driveshaft bolts to gain additional leverage while twisting off the nut. Remember, since the pinion gear is no longer in contact which the ring gear, it will spin freely making unbolting the pinion nut difficult.

The old saying that "tools make the job" is very true for this step. While you could unbolt this pinion nut before removing the ring and differential set, it would make "pushing" out the differential in Step 5 more difficult.

 
jig.jpg (75922 bytes)     Step 8: Remove the pinion bearing and place it onto the new pinion gear. This is where the job can get tricky. Randy Haywood has a hydraulic press and jig which is used to remove and replace the pinion bearing. If you do not have these tools available, getting the bearings off will take some creativity, or you may have to have a shop remove and install the bearing for you.

In this photo, you can see Randy placing the pinion gear bearing in a purpose made jig before placing the pinion in the hydraulic press. The new pinion gear (dry) is to the right.  The jig will be used to evenly pull the bearing off without damaging it, then reversed to press it back on the new pinion.

 
pressoff.jpg (91162 bytes)     Here is the bearing being pressed off the old pinion.

 
presson.jpg (83219 bytes)     And here is the new pinion being pressed onto the original bearings.

 
crush.jpg (53310 bytes)     Step 9: Put a NEW crush collar, which should come with the ring and pinion set, onto the new pinion and replace. Reverse Step 7 above to install. Torque the pinion nut to factory specs after re-installing the driveshaft flange. This photo shows a 4.56 pinion gear. If the backlash is not within factory specs, you will need to add or remove shims to the pinion before re-installing the pinion gear.


backlash.jpg (67402 bytes)     

Step 10: CRITICAL STEP.

Check the backlash after replacing the differential with the new ring gear installed. Replace the main bearing caps and torque them to factory specs. In this photo, a backlash tool is being used to check how much play there is between the ring and pinion teeth.

Factory specification is between 0.008 and 0.015 inches (to be confirmed). If the backlash is set too loose, there will be excessive gear whine. If the backlash is set too tight, the unit will operate very hot and could fail.  (I could have that reversed).

In order to adjust the backlash, shims must be added or removed from the pinion gear (Step 9) or ring gear carrier bearings to adjust. This could take lots of time since you will need to uninstall and re-install everything to make this work. I was fortunate that I only needed to adjust the pinion depth. Your luck may vary.

I am very glad I had a pro install my gears because this was BY FAR, the most time intensive part. Some differentials need no adjustment. Mine needed three tries and we set the backlash to 0.009 inches. Experience counts in this step.

My new ring gear and pinion were properly installed without changing the carrier bearing shim pack just by making a small correction to the pinion shims. Your results may vary. These results may be the case for others too, IF the carrier bearings were correctly shimmed from the factory and the gear manufacturing tolerances stack up favorably. Please refer to the Ford shop manual for the specific instructions on adjusting the ring gear and pinion shim packs from scratch for correct contact pattern and bearing preload procedures. Note however, that the shimming/adjustment procedures are identical to those used on the 8.8" straight axle cars.

 
    Step 11: Replace the differential cover using high temperature RTV sealant and re-install the unit into the IRS assembly by reversing the differential removal procedure.  Refill the differential with 75W90 gear oil before driving the car. This is usually done once the differential is re-installed on the car to avoid spillage.  Remember to add friction modifier if you are using conventional gear oil. Most synthetic gear oil does not require friction modifier, but you should confirm with the manufacturer of the oil.

 

Email me if you have any questions or feedback.

If you're in the North Carolina area, I highly recommend the guys at True Blue Performance. Randy Haywood, the proprietor, is one of the few people that I know of that specializes in not only Mustangs, but DOHC Cobras.  Some of the most competitive DOHC Cobras out there today have Randy's handiwork within. Check out their website to learn more.