up a differential is probably one of the most under-rated jobs that
a mechanic can tackle.
I have seen many "back yardmechanics slap new bearings in"
or "throw a new gear-set in" a rearend, and then take it to
a garage to let one of the technicians "set it up" so it won't
"Setting up" a rearend, involves so much more than merely
replacing parts, and installing them without any regard to the tolerances
that the "parts" must be held to.
Proper setup requires patience, cleanliness, and attention to detail.
Some Terms Relating to Differentials
Ring & Pinion gears
The driveshaft is connected to and drives the pinion gear, which in-turn
drives the ring gear at a reduced rate and at a 90 degree angle from
The ring gear is basically connected to the wheels.
This is the term used to describe the "clearance" between
the ring & pinion gears and is usually measured in thousandths-of-an-inch
by a dial indicator.
Example: .008 - .012"
Drive / Coast
The drive side of the ring gear is the side of the tooth that is contacted
when the vehicle is accelerating.
The coast side is the side of the tooth on the ring gear that is contacted
when the vehicle is decelerating (during engine-braking).
This refers to the tooth contact area of the gearset; In other words,
Where the Ring gear and the Pinion gear "Rub" together.
The Pinion bearings and Carrier bearings run a little tight, similar
to the way a wheel bearing should be "loaded".
The tapered bearings are adjusted to eliminate any play and then they
are set a little tighter, making them a little "hard to turn".
This is measured by checking the torque required to turn the bearings
with a dial torque wrench.
Example: 15 to 25 in.lbs.
This is set with "pinion shims" and controls how the pinion
contacts the ring gear in relation to the axle center.
A piece of "pipe" that is used to hold the pinion bearings
apart and offer some resistance for the pinion nut.
The pinion nut must be tightened down, crushing the crush sleeve, until
proper pinion preload adjustment is achieved.
or desirable pattern:
The drive pattern should be centered on the tooth.
The coast pattern should be centered on the tooth, but may be slightly
toward the toe.
There should be some clearance between the pattern and the top of the
or desirable pattern. The drive pattern should be centered on the
tooth. The coast pattern should be centered on the tooth, but may
be slightly toward the toe. There should be some clearance between
the pattern and the top of the tooth.
correct. Thinner pinion position shim required.
correct. Thicker pinion position shim required.
position shim correct. Decrease backlash.
position shim correct. Increase backlash.
Tools Needed For Proper Differential Overhaul
Ft.Lb. torque wrench
Dial In.Lb. torque wrench (0-50 In.Lb. or so)
Pullers and/or Press for removing and installing pinion and differential
Spanner wrench (for many drop-out carriers)
Compressed air, and 1/2" impact wrench is helpful
Gear teeth may have sharp edges. When handling gears, use care to avoid
Before removing the differential carrier from the housing, a pattern
must be taken if the original gearset is to be used again and to determine
how the gearset contact has been running.
This is accomplished by painting a number of teeth on the ring gear (both
the drive and the coast side) with a nondrying paint compound such as
Persian Blue or equivalent gear marking compound..
Have a friend "load" the carrier by prying between the differential
carrier and the rearend housing, making the pinion yoke hard to turn.
Turn the pinion yoke with a ratchet slowly and evenly, first in one
direction and then the other, through the painted area of the ring gear.
This will show you where the gear teeth are touching each other.
The gearset will "wear" a pattern that may or may not be serviceable.
A desirable pattern will be at the same height (or distance from the
Toe) on both the drive and the coast side, and slightly closer to the
Toe than the Heal side (see photo).
If the gearset shows a pattern tending towards the "desirable pattern",
the gearset may be reinstalled, but the original pattern must be adhered
Minor changes may be made to pinion depth and/or backlash to enhance
the pattern, but only as long as the final pattern has plenty of contact
If the gears are determined to be reusable, check backlash with a dial
indicator and note the reading.
Consult the manual for your application and compare with recommended
Mark the position of the differential bearing caps with a center punch
or letter stamp set to insure proper location and orientation during
Remove the differential carrier from the housing (it may be a tight
fit due to carrier bearing preload).
Note the position of any shims and their location.
Remove the pinion nut, and tap the pinion out of the housing (protect
the threads on the pinion; they can be easily ruined).
A 1/2" impact wrench is real handy for this, although you could
make a tool to hold the pinion yoke, but keep in mind that the pinion
nut will install with roughly 200 or 250 ft. lbs. of torque.
Remove the pinion bearing races from the rear-end housing.
Remove the carrier bearings from the differential carrier and the pinion
bearings from the pinion gear.
Inspect the rear-end housing closely in the area where the pinion bearing
races are pressed in and where the differential bearing races are "clamped
in" by the bearing caps.
Are there any signs of the races spinning or fitting loose?
How about on the bearing caps?
Check the differential carrier and the pinion gear for signs of loose
fitting bearings or spinning bearings.
If the rear-end housing does not check out, a replacement should be
found or the housing may be repaired by a reputable shop.
If the carrier of the pinion gear are found to be damaged, a replacement
should be found.
Install the bearing races into housing with any applicable pinion shims,
but make sure that there are no burrs in the housing that might hold
the bearing races up off of the bottom of the machined bore (sometimes
burrs are made when removing the old races).
Install the bearings onto the differential carrier, and again, check
for burrs on the carrier that might keep the bearing from being driven
all the way on.
If the ring gear was removed, check for burrs on both mating surfaces
where the ring gear contacts the differential carrier (run a flat wetstone
over both surfaces to remove any burrs) and reinstall the ring gear.
Torque the ring gear bolts to specification (you need a manual here),
and while your at it put some Loctite 271(RED) or 242(BLUE) on the bolts
just to make sure.
Install the pinion bearing on the pinion gear and don't forget any applicable
Install the pinion gear temporarily with the old crush sleeve (tap on
the side of the crush sleeve while it is on the pinion gear to make
it "longer" or "uncrushed").
Put some motor oil or gear oil on the bearings.
Install the outer pinion bearing and the pinion yoke (don't install
the new pinion seal at this time).
Put some motor oil on the pinion nut and tighten down until a preload
of the proper specification is achieved ( Stay on the loose side of
the specifications. i.e. 15-25 inch pound spec, use 15 inch pound).
Install the differential carrier (it should also be preloaded to specification,
so it will have to be tapped in), and torque the bearing caps to specifications.
Check the backlash and adjust to specification if necessary.
Remember how you pattern checked the gears when you first started?
Well, now you get to do it again.
If your pattern comes out "desirable", you're almost done.
If not, consult the picture for what changes to make to your setup.
Make those changes, reinstall, pattern check, and consult the picture
for what changes to make to your setup. . . . well you get the idea.
You have to have patience!
Keep at it until you are satisfied.
This is the place where most people fall short of proper setup, and
blame the gear noise on a "noisy set of gears".
If the pattern looks good, then there won't be any excessive noise.
Note that all gears make an acceptable amount of noise.
Rearends, transmissions and engines all make noise.
It is up to the rubber engine, transmission, spring, and body mounts
to insulate those noises from the passengers, so don't overlook these
items when chasing excessive noises.
After you are satisfied with the pattern, disassemble the rearend and
install a new crush sleeve, and reassemble (don't forget the new pinion
Reinstall the old pinion nut with motor oil and tighten until all play
is eliminated in the pinion gear (but not preloaded yet).
Remove the pinion nut and clean threads on pinion gear and new pinion
nut with carburetor cleaner, and install nut with Loctite 271(red) or
242(blue) and tighten down until proper pinion preload is obtained.
Install the differential carrier, and torque the differential bearing
caps (it wouldn't hurt to use Loctite on the cap bolts).
Pattern check one last time (hopefully), to make sure that you reinstalled
everything properly and check the backlash one last time.
If within specification, your done with the setup.
Install the axles, tap the rear cover surface flat (if yours has one).
Now, if your cover is a heavy duty one with stiffening ribs, reinstall
with a gasket painted with "brush-on" Aviation Permatex or
equivalent and snug cover bolts moderately.
Otherwise, if your cover is flimsy (or worse, a cheap chrome one), install
with no gasket and use Permatex Ultra-Black or equivalent.
If you insist on using regular silicone with a gasket (Lord knows why),
go and install bath-tubs or caulking around household windows, and let
somebody else work on your car!