as there was no manual encluded I can only guess, that the 2x4 washers included with each set of 2 belong between the bearing and the hub so tightening the nut will press the hub against the bearing instead of pressing it against the pin.
but this will put a rather high axial force against the bearings so they will break very soon (normal ball bearings arn't designed to support this kind of force).
leaving them out will press the hub against the pin, result in some deformation in the hub.
so.. how do you do it?
and how is this done with the 17mm ones that traxxas sells?