These universal mounting hubs are designed to work with most 4mm shafts, including round and "D" shafts. Each hub has four mounting holes for 4-40 screws (not included), letting you mount custom wheels or mechanisms to your motors. 4-40 hex set screws (one 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.