The shot with the blue tape over the oil trough shows the gap between the outer end of the crossbox and the inner end of the spacer. If this gap were not filled with a thrust collar, the gears would have a tendency to become disengaged this amount: either the axle would tend to migrate to our left, or the crossbox would get shoved to the right, neither of which is good for gear life or the proper tramming of the truck.

Photo by Bill Liebman