These universal mounting hubs are designed to work with most 8 mm diameter shafts, including round shafts and “D” shafts. Each of the two included hubs has six mounting holes for M3 screws (not included), letting you mount custom wheels or mechanisms to your motors. The four included M3 hex set screws (two for each hub) allow secure coupling of shaft to hub, and a 1.5 mm hex wrench is included for use with the set screws. A dimension diagram of the hub is available under the resources tab.