I have a set of ACS mag wheels that I pulled off an old Diamondback years ago. I'm wanting to re-use them on a bike that I'm putting together for my daughter. I'm having issues with the freewheel/axle/hub assembly. My giant disclaimer is that up to now I have only built road bikes for myself, I'm new to BMX. The problem that I'm having is with the freewheel assembly that is different than the typical one that threads on to the hub. The one that I have threads on to a cone nut that is slid through the center of the hub. Once tightened it is supposed to hold the freewheel in place, allowing it to spin but still keeping tension when you pedal. However I seem to be missing a piece because as I ride the cone nut works itself loose allowing the assembly to spin freely when I pedal. There are 4 concave shims that I thought would keep the pressure but they just flop around doing nothing. Pardon the picture quality, I used my cell phone.

Wheelset in question:


Rear wheel:


Where freewheel and bearings go, cone nut is slipped through other side of wheel and is held tight from inside:


Axle with bearings and freewheel:


Freewheel assembly with cone nut tightened:


Same as above but with cone nut loose. This is what happens as you ride:


The 4 shims that seem to do nothing:


What am I doing wrong and/or what am I missing?