a spacer that goes in between those 3/16 x 3/8 bearings.
I helped a buddy convert an RTR T3 over to CVD's and the procedure was the same for my FT GT1.
7377 is the part number.
I got it right from my B4 manual.
If you didn't put that spacer in there, the next thing that would happen is you would be breaking axles.
I'll send you a bill for all my help