This is a side clamping collar. Use the smallest standard size screw. I was frustrated with the metal collar and the set screws always getting stripped, stuck, lost etc...The "small" shaft collar has flat surfaces to accommodate a locking nut on one side. Still has clearance for the VEX motor.This part can fit between the 393 motor and support structure. IF you use the smallest screw, it will have proper clearance to turn. Make sure it is fully inserted so it does not jam. Enjoy.