How to Install a Yukon Gear Kit on your Ford Mustang

Disassembly
1. Ensure that you have everything you need before working. Verify whether all new parts
match the application and parts invoice.
2. Lift the vehicle using an appropriate lift or jack, and support the vehicle with approved
jack stands. Never work on an unsupported vehicle.
3. Drain used gear oil into a suitable container. We recycle our waste oil and request that
you recycle yours, too.
4. Remove the axle shafts.
5. Mark the main caps for reinstallation on the appropriate side and in the correct orientation.
6. Mark all shim thicknesses and locations during disassembly.
7. Thoroughly clean and inspect all parts.

Order of Adjustments
Differential assembly and setup adjustments include:
1. Pinion Depth
2. Pinion-Bearing Preload
3. Backlash
4. Carrier-Bearing Preload

Shim Selection
For a baseline, reassemble the differential with the shim combinations that were used in
the differential during the previous assembly.

Parts Preparation
Clean all parts, including all new components, with solvent or brake cleaner, and thoroughly
wash out the axle housing’s interior. Ensure that the oil passages that feed the pinion and
the grooves behind the carrier bearings are free from any metal particles.

Seal Preparation
Polish all seal surfaces with fine-grit emery cloth or sandpaper. Wipe all surfaces with a
clean rag dampened with either fresh oil or solvent to remove any metal particles.

Coat bearings and seal surfaces with gear oil and seals with lithium grease

Assembly Oil
When assembling internal components, coat all bearings, and seal surfaces with fresh gear
oil. Never use bearing grease on pinion or carrier bearings; it will negatively influence
assembly readings. Coat all seals with grease, preferably white lithium grease. Use clean
gear oil if white grease is not available.

Pinion Trial Assembly
Assemble the pinion with its original shims yet without a crush sleeve to establish an
approximate pinion depth. When installing the pinion, tighten the nut slowly until it reaches
preload specifications.

Initial Carrier Assembly
Install the assembled gear carrier with its ring gear into the housing. It is easier to remove
and replace the carrier during trial assemblies if the carrier bearing preload is fairly snug
instead of tight.

Checking the Pattern
We can determine how gears mesh by changing how close the pinion gear is to the ring
gear centerline. While we can’t see how the gears actually relate to each other, we can coat
their mating surfaces with gear-marking compound and read the patterns the gears create
as they mesh.
Genuine gear-marking compound offers a clear indication of gear contact without running
or smearing. Anything other than gear marking compound (such as blue machinist
dye) will not give a clear indication of tooth contact. Dilute the marking compound with a
small amount of oil if necessary to create a smooth, but not runny, paste. Coat three or
four ring-gear teeth in at least two places with a moderate amount of compound and rotate
the ring gear four or five times around the pinion gear in both directions. Rotate by grabing
and turning the ring gear, not the pinion.
Pinion resistance against the rotating ring gear helps establish a good pattern. Pinion
bearing preload usually provides enough resistance for a good pattern, but additional
resistance can be added by wrapping a shop towel around the yoke and pulling the two
ends tight.

Important Pattern Information
The pattern’s position to the tooth face (ridge) and flank (valley) notes the pinion depth.
Disregard the pattern’s position to the tooth’s heel (ring gear outside diameter) or toe (ring
gear inside diameter).
Gear patterns change from heel to toe, but in most cases an ideal heel-to-toe pattern is
impossible to achieve. Furthermore, the housing itself influences the heel-to-toe pattern
and the pattern cannot be changed without machine work. Trying to obtain a pattern centered
exactly between the heel and toe usually leads to frustration and a noisy gear set, if
the face to flank pattern is not correct.
Instead, concentrate only on the position of the pattern and how it relates from face to
flank of the ring gear teeth.

• A contact pattern centered from face to flank indicates the correct pinion depth.
• A contact pattern closer to the gear face means the pinion is too far away from the ring gear. To correct the pattern, move the pinion toward the ring gear centerline.
• A contact pattern closer to the gear flank means the pinion is too close to the ring gear.

To correct the pattern, move the pinion away from the ring gear centerline.

Used Gear Sets
When setting up a used ring and pinion, concentrate only on the pattern created on the
coast side of the ring gear teeth. Pay little attention to the drive side. This is true for most
used gear sets, although both the coast and drive sides should be considered in some
instances.

Adjusting Pinion Depth
When changing the pinion depth, make large changes until the pattern is close to ideal.
Consider 0.005” to 0.015” a large change and 0.002” to 0.004” a small change. Intentionally
make adjustments that move the pinion too far at first.
If the pinion moves too far and the pattern changes from one extreme to the other, the
correct pattern lies somewhere between the two extremes. Once you get close to the correct
pinion depth, make smaller changes until the pattern centers between the face and the
flank of the ring gear teeth.
Once the backlash and pinion depth meet tolerances, remove the carrier and establish
the final pinion bearing preload.

Pinion Bearing Preload
The pinion bearing preload is related to the amount of force the pinion nut exerts on the
pinion and its bearings. Axle builders generally measure the pinion’s preload by rotating
the pinion gear by its nut with a pound/inch-graduated torque wrench.
Axles generally use crush sleeves or shims to set pinion preload. Despite the difference,
one thing remains the same: oil the pinion nut washer surface during all assembly procedures
and apply medium-strength thread-locking compound (red) to the pinion nut
threads during final assembly.
A lubricated washer reduces friction and prevents the nut and washer from galling during
tightening procedures. The thread-locking compound helps retain the nut position and
pinion bearing preload.
Before setting the final pinion bearing preload, install the pinion seal, slinger (if applicable),
and crush sleeve (if applicable).

Preload Shim Design
Setting pinion bearing preload in differentials that use shims exclusively often requires several
attempts.
Carefully clean the shims, as dust and metallic particles on the shim surfaces cause false
preload readings. Start the procedure with the original shims, or add 0.003” to the original
shim-pack dimension to compensate for the amount the bearings settled into the housing.
Tighten the pinion nut slowly to avoid damaging the bearing should the shim stack prove
too thin. The pinion nut torque varies by thread size but is usually between 200 and 300
pounds/feet.
Measure the preload by rotating the pinion with the pound/inch torque wrench. If the
preload doesn’t meet the axle manufacturer’s specifications, change the shim pack dimension
in one of the following ways:

• Remove shims to increase pinion bearing preload.
• Add shims to decrease pinion bearing preload

Write down each shim-pack combination and its resulting preload when changing shims.
After reaching the correct pinion-bearing preload, moderately tap both ends of the pinion
to seat the bearings, races and yoke. Do not strike the pinion too hard, as excessive force
may damage the bearings. Carefully double check the preload after seating the bearings.

Final Backlash and Carrier Bearing Preload Adjustments
After setting the pinion depth, backlash, and the pinion bearing preload, set the carrier
bearing preload. Three different shim or adjustment methods, screw adjusters, outside
shim design, and inside shim design, cover most differentials.

Screw Adjuster Design
The easiest method uses screw adjusters to set the backlash and carrier bearing preload.
Carefully oil the adjuster threads on both the housing and on the adjusters themselves.
Note the order in which you tighten the adjusters so the backlash remains consistent, even
when subjected to heavy loads.
The pinion gear always forces the ring gear away from itself whenever it transfers power.
Start with a looser backlash setting than the axle’s manufacturer calls for and always
ensure that the last adjustment made to the left adjuster tightens it.
If the backlash becomes too tight, loosen the left adjuster first, followed by tightening the
right adjuster. To reestablish the backlash setting, tighten the left adjuster. Ensure that the
last adjustment made to the left adjuster tightens it to eliminate any space between it and
its bearing race. Any space or looseness on the left side will let the carrier deflect when
under load, and this will allow backlash to open up when driven under load.
Once the backlash approaches the manufacturer’s recommendations, tighten both left
and right adjusters evenly to increase carrier bearing preload. Set the carrier bearing preload
to approximately 150 to 200 pounds/feet. It is difficult to damage bearings with excessive
carrier bearing preload on a screw-adjusted differential.

• If the preload is close and the backlash is too loose, tighten the left adjuster a notch or two until the backlash is correct and the preload is sufficient.
• If the preload is close and the backlash is too tight, tighten the right adjuster until the backlash is correct and the preload is sufficient.

As stated before, ensure that the last adjustment made to the left adjuster tightens it. That
will eliminate the possibility of a space between the adjuster and the bearing race.

Outside Shim Design
This design uses shims between the carrier bearing races and the housing. Initially set the
backlash with very little carrier bearing preload. After setting the backlash, add equal
amounts of shims to both sides of the carrier to set the carrier bearing preload as tight as
possible without damaging the shims (carrier bearings in this axle design hardly ever fail
due to excessive carrier bearing preload).

• If the preload is close and the backlash is loose, add shims to the left side. This increases the carrier bearing preload and tightens the backlash at the same time.
• If the preload is close and the backlash is too tight, add shims to the right side. This

increases both the carrier bearing preload and the backlash at the same time.

Inside Shim Design
This design uses shims between the carrier bearing and the case. Initially set the backlash
tight and the preload light, as it will make carrier removal and installation easier. After setting
the backlash, add equal amounts of shims to both sides until the correct preload is achieved.

• If the preload is close and the backlash is loose, add shims to the left side. This increases the carrier bearing preload and tightens the backlash at the same time.
• If the preload is close and the backlash is too tight, add shims to the right side. This

increases the carrier bearing preload and the backlash at the same time.

Pattern
Now that the pinion depth, pinion bearing preload, backlash, and carrier bearing preload
are set, check the pattern one last time to make sure that it is correct.

Oil
When filling the axle with oil, use a high quality name brand and fill the unit to the manufacturer’s
recommended capacity. Synthetic oil is recomended for most applications.

Checking Backlash
Backlash is the free movement of the ring gear with the pinion gear fixed in place. All ring
and pinion sets are matched pairs. Make sure you have a matched ring and pinion set.
Clean all parts before you start assembly. Apply a light coat of oil to all bearings. Examine
all components and remove any burs, nicks or sharp edges that could cause components
seat improperly.
1. Set backlash to proper clearance. (See page 11)
2. Shim or adjust the ring gear away from or closer to the pinion to get the correct backlash.
3. The pinion bearing preload should match those recommended on the specification
sheet (See page 2). Achieve the recommended preload with a preload shim pack (combine
shims of various thicknesses) or a collapsible crush sleeve. Use a new crush sleeve
during final assembly.
4. Achieve the pinion depth by shimming the pinion toward or away from the ring gear
centerline. Start with the same shim thickness on the new gear set as on the old set.
Note that not all housings are shimmed in the same location.

1. Normal or desirable pattern: The pattern should be centered on the tooth from face to
flank. There should be some clearance between the pattern and the top of the tooth
(face) and between the pattern and the bottom of the tooth (flank).

2. Pinion is too close: Move the pinion away from the ring gear centerline

3. Pinion is too far away: Move the pinion towards the ring gear centerline.

Pattern Movements Summarized
1. Move the ring gear closer to the pinion to decrease backlash.
2. Move the ring gear farther from the pinion gear to increase backlash.
3. Move the pinion closer to the ring gear to move the drive the pattern deeper on the
tooth (flank contact) and slightly toward the toe. The coast pattern will move deeper on
the tooth and slightly toward the heel.
4. Move the pinion further away from the ring gear to move the drive pattern toward the
top of the tooth (face) and slightly toward the heel. The coast pattern will move toward
the top of the tooth and slightly toward the toe.

New Gear Break-In
All new gear sets require a break-in period to prevent overheating damage. Any overloading
or overheating will break down the gear oil, which will cause the ring and pinion to fail.
This can be determined by inspection and will void the gears’ warranty.
Please follow these guidelines to insure proper break-in.

• Avoid heavy acceleration during the break-in process.
• Drive the vehicle lightly for the first 15 to 20 miles and stop. Let the differential cool

before proceeding.

• Avoid heavy acceleration for the first 100 miles.
• Drive the vehicle at least 500 miles before towing to retain the gears’ warranty.
• When towing for the first time, drive for a very short distances (less than 15 miles) with the full load and stop. Let the differential cool for about 20 minutes before proceeding.

Repeat this procedure two more times (45 miles total) to fully break in the gears.

• Change the oil after the first 500 miles. This will remove any metal particles and phosphoric coating shed by the gear set during the gears’ break-in period.

These towing instructions may seem unnecessary to most people, but we have seen
many differentials damaged from being loaded before the gear set was fully broken-in.