The main purpose of having the diff covers is to keep small stones and pebbles from jumping into the diff housing as that would chip or strip your diff's main outer gear as well as wreak havock on your belt. As the 511 was originally designed, the outer gear of the diff is the only part of the diff that is exposed on top by the uncovered diff housing. The internal gears (or balls and washers on the ball diff) are all pretty much sealed away from the outer elements. I have no honest answer for why the hell the designers at Tamiya designed the 511 with exposed diffs. It makes no sense to me, but thank God for Fred coming to our rescue.
