Don't Let Her Rear End Scare You

by Dick Zakrzewski

Set the pinion depth

Pinion depth is the distance between the centerline of the ring gear and the nose of the pinion (see Sketch #1).

Measure from surface "A" of the differential carrier to the lowest point of the differential side bearing seat with a depth micrometer. See Photo #1 and "R1" in Sketch #2. Also measure depth to the lowest point of the side bearing cap. See Photo #2 and "R2" in Sketch #2. Add the two micrometer readings together, divide by 2 and you have the radius of the differential side bearing seat. See "R3" in Sketch #2.

(R1 + R2)/2 = R3

Sketch #1

Sketch #2

Photo #1

Photo #2

Note: The actual location of the ring gear centerline is not likely to be at the interface of the side bearing cap and differential carrier (surface "A"). Hence, you must determine the amount of this "centerline offset".
Calculate the "centerline offset" by subtracting "R1" from "R3".

Centerline offset = R3 - R1

Note: "Centerline offset" can be a negative number if "R1" is greater than "R3". Remember, it is also possible that the "centerline offset" could be zero, but this is not likely.
Calculate "current pinion depth". To get this dimension, measure the distance from surface "A" to the nose of the pinion gear-call it dimension "P". See Photo #3 and Sketch #2. "Current pinion depth" equals "P" plus the "centerline offset".

Current pinion depth = P + centerline offset

Photo #3

Note: If the "centerline offset" is a negative number ("R3" less than "R1") then subtract it from "P".
Look at the end of the pinion gear and record the dimension etched onto the gear. It should read something like 1.201, +0 (your number may be a little different). This dimension I call the "target pinion depth". See Photo #4 and Sketch #1.

Note: Aftermarket gears like those produced by Richmond are run-in at the settings etched on the pinion and ring gears.

Photo #4
Compare the "current pinion depth" with the "target pinion depth" taken from the end of the pinion. The difference is the "pinion depth shim thickness".

Target pinion depth - current pinion depth = pinion depth shim thickness

If "current pinion depth" is 1.193" then the shim thickness is .0075".

1.201" - 1.193" = .0075"
Combine shims to give the correct thickness. See Photo #5

Note: The +0 etched on the pinion gear means that the "target pinion depth" can be a little less (a couple thousands of an inch) but not greater, or the shims can be a little thinner but NOT thicker.

Photo #5
Install the shim(s) between the drive pinion retainer and the differential carrier. See Photo #6. Re-torque the pinion retainer bolts.
Verify the shims you selected give the desired results. Measure again from surface "A" to the nose of the pinion. This new dimension plus the "centerline offset" should equal the dimension etched on the nose of the pinion - in this case, 1.201" (or a couple thousands of an inch less). This process will get you very close to the "target pinion depth". Make small changes to the shim package to get closer to the "target pinion depth" if necessary.

Photo #6