Differential Drive Pinion and Drive Gear


Whenever the ring and pinion gears or the pinion or differential case bearings are replaced, pinion gear depth, pinion bearing preload, and the ring and pinion gear (another name for the drive gear) tooth patterns and backlash must be checked and adjusted. This holds true for all types of differentials except most front-wheel-drive differentials that use helical-cut gears and taking tooth patterns is not necessary. Nearly all other final drive units use hypoid gears that must be properly adjusted to ensure a quiet operation.

Pinion Gear Depth

Pinion gear depth is a measurement of how far the pinion gear extends into the differential housing The measurement can vary according to where the pinion bearing race is position in the case. During manufacture, if more was machined from the case or if the ledge on the pinion gear had more or less metal on it, pinion depth will vary. This adjustment requires a special tool when starting from scratch without existing gears.

A special tool is used to adjust pinion depth.

Pinion gear depth is adjusted with shims that are placed behind the pinion bearing in the housing.

Typical placement of a pinion gear depth shim.

The thickness of the drive pinion rear bearing shim controls the depth of the mesh between the pinion and ring gear. To determine and set pinion depth a special tool is normally used to select the proper pinion shim.

Special tool for measuring proper pinion gear depth. Reprinted with permission.

Always follow the procedures in the service manual when setting up the tool and determining the proper shim.

If a gearset is produced that will result in a depth that is different than stock, there will be a number on it that will tell how much compensation to make when setting up the clearance between the two. A +2 on the pinion gear means that the shoulder for the shims should be 0.002" too large for stock. This calls for a shim that is 0.002 less than the shim that was on the old gear (if that gear had a "O" mark).

Pinion depth shim mark.

Shims vary in thickness. Use a macrometer to double-check the size of labeled shims. If a pinion depth gauge is not available, try checking the gear tooth pattern while using a shim of half the thickness of the maximum shim specification. Tighten the pinion nut to 25 foot-pounds for the preliminary test so you do not ruin the crush sleeve.

Pinion and Ring Gear Tooth Pattern

To check the gear tooth pattern, paint several ring gear teeth with nondrying Prussian blue, ferric oxide, or red or white lead marking compound. White marking compound is preferred because it tends to be more visible than the others are. Use the pinion gear yoke or companion flange to rotate the ring gear. This will preload the ring gear while it is rotating and will simulate vehicle load. Rotate the ring gear so that the painted teeth contact the pinion gear. Move it in both directions enough to get a clearly defined pattern. Examine the pattern on the ring gear and make the necessary corrections.

Most new gearsets purchased today come with a pattern prerolled on the teeth. This pattern provides the quietest operation for that gearset. Never wipe this pattern off or cover it up. When checking the pattern on a new gearset, only coat half of the ring gear with the marking compound and compare the prerolled pattern.

Commonly used terms for describing the possible patterns on a ring gear with the recommended corrections.

Measuring and Adjusting Backlash and Side Bearing Preload

On a Final Drive Assembly with a Shim Pack

  • Measure the thickness of the original side bearing preload shims.
  • Install the differential case into the housing.
  • Install service spacers that are the same thickness as the original preload shims between each bearing cup and the housing.
  • Install the bearing caps and finger tighten the bolts.
  • Mount a dial indicator to the housing so that the button of the indicator touches the face of the ring gear. using two screwdrivers, pry between the shims and the housing. pry to one side and set the dial indicator to zero, then pry to the opposite side and record the reading.
  • Select two shims with a combined thickness to that of the original shims plus the indicator reading, then install them.
  • Using the proper tool, drive the shims into position until they are fully seated.
  • Install and tighten the bearing caps to specifications.
Art courtesy of Delmar - Thomson Learning.
  • Check the backlash and preload of the gearset. Check the backlash by rocking the ring gear and noting the movement of the dial indicator. Adjust the shim pack to allow for the specified backlash. Recheck the backlash at four points equally spaced around the ring gear.