Most pinion nuts get tightened until there is a specific amount of drag, called "preload" as the yoke is turned. Those ones use "crush" sleeves between the inner and outer pinion bearing cones, and there is way too much clearance when the sleeve is new. It takes a massive amount of torque to get the sleeve to begin shortening itself (crush). Once the crush sleeve allows the bearing cones to contact the outer races (called the cups), a slight amount more tightening produces the necessary drag. A crush sleeve is a one-time use item, so get a new one anytime you're building up a ring & pinion set. Most pinion nuts used in that application don't lock with a cotter pin, however. They will either have a nylon self-locking insert, or they just hold their position from being very tight.
If the pinion preload is set with shims instead of a crush sleeve, you'll probably have a hole or two in the pinion shaft for a cotter pin, and the nut needs to be TIGHT! Don't back the nut off to align with a pin hole, either tighten it some more, or file the castle nut slots enough that the pin will fit one of the holes. Red LocTite is also your friend! Normal pinion preload for a passenger car or light truck will be somewhere in the 12 to 25 inch-pound range to keep the pinion gear turning witn no other drag such as the ring gear, axles, etc.
Jerry
