[magnets]

Hex shafts and hex bearings are never going to be perfectly sized. There's a reason nobody uses hex bearings outside of FRC, and why McMaster sells the unpolished "mill" finish slightly oversize hex shaft.

To get a perfect fit, make sure that the bearing hole in the frame is perfect, and turn down the end of the hex shaft and use normal round bearings. Then, for the wheel, use a vex hex shaft and vex hub. They work real well together.