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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.
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Step 2: Drain differential
fluid. The fluid is foul smelling so work in a well ventilated area.
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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.
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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.
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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.
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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.
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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.
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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.
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Here is the bearing being pressed off the old
pinion.
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And here is the new pinion being pressed onto
the original bearings.
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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.
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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.
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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.
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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.
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