Ford 2003 Mustang Workshop Manual page 336

2. Install the bearing caps.
3. Install the four bolts.
11. NOTE: Use only flat, clean drive pinion bearing adjustment shims (4663).
NOTE: Selection of too thick a shim results in a deep tooth contact at final assembly. Do not
attempt to force the shim between the special tools. A slight drag indicates correct shim
selection.
Use a drive pinion bearing adjustment shim as a gauge for shim selection.
After determining the correct shim thickness, remove the special tools.
12. Position the correct thickness drive pinion bearing adjustment shim and the differential pinion
bearing on the drive pinion gear. Using a suitable press and the special tools, press the
differential pinion bearing until it seats firmly against the drive pinion gear.
13. Place a new collapsible spacer on the pinion shaft against the pinion stem shoulder.