The outter and inner bearings are both larger on the RPM hubs:
http://www.rpmrcproducts.com/product...3.htm#trutrack
inner bearings are 12x21mm, outer are 6x15mm ( can't remember stock sizes ). The only cvds that will work are ones designed for the new emaxx models- stuff designed for the old 3906 emaxx won't work as it used inner & outer bearings that were the same size ( new emaxx shafts use totally different size inner and outer bearings and a different style of stub-axle ).