Bigred's got the right idea; it's a bolt with a nut much like many 1/8th scales use. Specifically, it's a Lunsford screw originally meant to be a Losi transmission bolt. It has a long shoulder that the hub pivots on, and the threads start just where the screw comes out of the arm
. The nut on the front helps strengthen the control arm, as loads on the wheel are distributed through the arm better.
Unfortunately, BK has bent some of the Ti ones, so we're looking into other solutions.