when doing a rebuild on the differential, correct wear pattern must be determined for best results. We do not want gear noise or rear end noise caused by improper teeth contact between the differential side gears and the pinion gear teeth. We paint the pinion with machinist bluing ( cheap blue dye) and rotate the assembly. we then check wear pattern on the gear face. If not correct we shim the pinion to achieve proper pattern.
We also have carrier shims that adjust backlash. Both the pinion and carrier have a Nominal Mounting distance or the ideal dimension but due to manufacturing imperfections in the gear making process the ideal and the real world dimensions will almost never be the same so we use the bluing method to check wear pattern. By adding or removing shims we adjust the contact depth ( pinion shim) and backlash ( carrier shim) to obtain optimum gear contact.
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