Product information "Pololu Universal Aluminum Mounting Hub for 3mm Shaft, #2-56 Holes (2-Pack)"
These universal mounting hubs are designed to work with most 3 mm diameter shafts, including round shafts, “D” shafts, and Tamiya’s 3 mm hexagonal shafts. Each of the two included hubs has four mounting holes for #2-56 screws (not included), letting you mount custom wheels or mechanisms to your motors. The two included #4-40 hex set screws (one for each hub) allow secure coupling of shaft to hub, and a 0.05″ hex wrench (often the smallest size in SAE Allen wrench sets) is included for use with the set screws. A dimension diagram of the hub is available under the resources tab.