This 3/8" Diameter ball caster has a durable black ABS housing, it contains a 3/8" Diameter white plastic ball, also included are two acrylic spacers (1/16″ and 1/8″ thick), and two #2 screws.
The total height of the ball caster can be adjusted from 0.4 to about 0.6″ using the included spacers. The included #2 screws can be used to mount the ball caster to your chassis (the distance between the screw holes is 0.53″).
Note: The ball casters are designed for small robots; they are not intended to bear more than a pound. With more weight than this, friction between the ball and the housing will keep it from rolling well.