What’s the big deal about shims? The pinion-bearing adjustment shim (official Ford terminology!) controls how far into the case the pinion gear fits, and the differential bearing shim controls the lateral (side-to-side) location of the ring gear. If this relationship is off, you can end up with noisy gears and/or broken teeth because of incorrect or insufficient contact between the ring-and-pinion gear teeth. Here, Moore is checking lash, the amount of free movement between the ring-and-pinion teeth (i.e. how far he can turn the ring gear while holding the pinion stationary). The spec is 0.010 to 0.012 inch.