Product information "Pololu Universal Aluminum Mounting Hub for 1/4" Shaft, #4-40 Holes (2-Pack)"
These universal mounting hubs are designed to work with most 0.250 inch (6.35 mm) shafts, including round shafts and “D” shafts, such as those on our NEMA 23 stepper motors. Each of the two included hubs has four mounting holes for #4-40 screws (not included), letting you mount custom wheels or mechanisms to your motors. The four included #4-40 hex set screws (two 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.