When you tighten the nut, your pushing against the flange, which in turn pushes against the inner race of the outer pinion bearing, which pushes against the spacer/sleeve, which pushes against the inner race of the inner pinion bearing, which pushes against the shims, then the pinion. Since the outer bearing is not a friction or interferance fit, but a slip fit, if you over crush the crush sleeve by hitting the flange, you lose the compression on the inner race of the outer bearing. The race is being pinched between the flange and the spacer/sleeve. This will let the race spin on the pinion shaft. Do I need to explain further than this? Or is it still too general? Or am I not using my 20+ years as a mechanic and just repeating what some one else said? If your just using your rig on the street or mild wheeling (not implying anything <IMG SRC="smilies/biggrin.gif" border="0"> ) then by all means use the crush sleeve. But I also beleave that "an ounce of prevention is worth a pound of cure". I like to give myself every edge in playing hard, and getting home.