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 M3 set screws for securing the hubs to motor shafts, and one 1.5 mm Allen wrench for use with the set screws. Each hub has four threaded mounting holes for M3 screws (not included).
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more info: http://www.pololu.com/product/1996