Hi, I'm a mechanic learning about setting up differentials of the typical/basic type; open differentials with ring and pinion gear.  Nothing fancy.

I am not sure I understand why there are pinion depth shims on the pinion shaft between the cone bearing and the pinion gear.  But because there is a different amount of shimming on each differential I've seen I am guessing that there is a variance in the manufacturing process that is being compensated for.

I had thought maybe the pinion gear itself was the variable, but after extensive reading I've found a couple sources that say it is the housing that is the cause.

So my questions are:

Q1: what is the manufacturing process for the housing?  

(ie cast from ductile iron, line bored for carrier bearings, bored for pinion bearings?)

Q2: What are the shims compensating for exactly?

Q3:  Is there manufacturing variation in the depth of the pinion gear, ie the toothed portion?

Q4:  Is there variation in the total depth of the pinion gear and shims?

I know that there are shims correcting a variation and that the correct pinion depth is important for proper gear contact but I have no idea which dimension is variable and which ones should be constant.

I'm sort of mind boggled as to whether the shimming stays with the housing or with the gears.  It's often marked on the pinion gear but some sources claim it is specific to the housing, and the usual recommendation seems to be to transfer the old shimming to the new gears when doing a replacement.

Thanks for your help, apologies for any ambiguities about my inquiry and my long winded explanation.

-Andrew