These universal mounting hubs are designed to work with most 6mm shafts, including round and "D" shafts.Each hub has six mounting holes for 4-40 screws (not included), letting you mount custom wheels or mechanisms to your motors.4-40 hex set screws (two included for each hub) allow secure coupling of shaft to hub.A 0.05" hex wrench (often the smallest size in SAE Allen wrench sets) is included for use with the set screws.