Here is a pic of 5mm to 8mm tapered, self-centering rod couplers that I printed in Polymax PLA. When I (finally) receive my bed upgrade, I intend on replacing the stock ROBO couplers and see how well these work. The will fit any 5mm stepper motor shaft and either 8mm or 5/16ths threaded rods. The tapered nuts over the tapered threads both center and firmly attach the z rods to the motors. Please disregard the quality of the print as that ribbing is just what the couplers are designed to minimize. If anyone is interested, I will post the .stl files for both parts. It was originally designed by "loco" and posted on Thingiverse as a 5mm coupler on either end. I just modified it to fit our ROBO z rods. It seems to hold really well in testing but I will probably either glue or tap a set screw for the actual fitting.