Backlash is measured as the amount, in inches, that a ring gear tooth can move, total, when installed and the pinion is held stationary. It is a linear measurement of rotation, but it works. It is the clearance between the two sets of teeth. Final adjustment is by moving the ring gear left or right to increase or decrease that clearance. That moves the the root of the ring teeth closer to the tip of the pinion teeth. Since they are v shaped, the clearance between them changes. .008 to .012 in or so is ok, from memory.
Pinion depth is nominally described as the distance from the axle centerline to the flat rear surface of the pinion gear. Note that the pinion shaft centerline does not intersect the axle shaft centerline.
It is usually measured indirectly, using a tube or machined round bar that fits exactly in the bearing caps, with a hole for a measuring tool, or a similar setup with machined spacers, used as gage blocks. Since all of those dimensions are known, you can check the distance from some surface to the pinion and calculate its depth. This country boy uses a machined piece of angle across the bearing caps and the extension end of a dial caliper.
You cannot easily measure it with actual parts installed.
The crush collar takes two extremes, something like 150 to 200 or more ft-lbs to crush and tighten, and you measure how much you have tightened it by the amount of torque in in-lbs it takes to rotate the pinion (without the ring gear installed) after tightening. 12 to 20 in-lbs or so, I forget the actual spec. I use a 3 ft pipe on the breaker bar on the pinion nut, and a small postal/fish scale on a wrench handle to measure in-lbs.
