You can't make a blanket statement that they will break in just fine building as described in the manual. There are always minor differences in the diff case. I'm building one right now and neither the front or back will be shimmed anywhere close to the manual suggestion. On the rear I had to pull the shim on the crown gear side. It's tight but I know that one will wear right in. For the front I'm gonna end up with 2 or maybe 3 shims opposite the crown gear, none on the crown side and none on the pinion just to get it to spin at all. The front diff fit is silly tight.