I don't know why they keep putting those o-rings in the kits. I built 3 MBX5's & Prospecs for me & 2 for others. Never used them. Another tip you should use is too put a small bead of grease all the way around the opening in the bottom of the outer diff cases before you bolt them down to the chassis. Some will squish out the sides on the chassis when you bolt it down, just wip that off. This trick will keep all the dirt out & help your ring & pinion gears stay clean & last forever.