The one-piece hub/sprocket is made from chrome-moly steel and finished with black oxide. A steel key locks the wheel and sprocket together, so the torque on the sprocket gets transferred to the high-traction rubber tire. One bronze and one ball bearing go into each wheel. The bore diameter is 1/2-inch. These sprockets are made to be used with #35 roller chain (3/8" pitch). The width of the tread is 1.5 inches. These rugged wheels are ideal for robotics and other industrial and mobile applications. Other sizes, including 3, 4, 5, 6, 8, 10, and 12.5-inch wheels, are also available. Search for "BattleKits Wheels".