Pololu Universal Aluminum Mounting Hub for 3mm Shaft, #2-56 Holes (2-Pack).
These universal aluminum mounting hubs allow you to mount custom wheels and mechanisms to 3 mm diameter motor shafts. The set includes two hubs, two #4-40 set screws for securing the hubs to motor shafts, and one 0.05″ Allen wrench for use with the set screws. Each hub has four threaded mounting holes for #2-56 screws (not included).