This Swivel Caster has double-ball swivel bearings for the smoothest swiveling. They have 3" diameter polyolefin wheels for smooth rolling and have an overall height of 4". These are hollow kingpin casters and are mounted using a 1/2" diameter bolt and nut (included). 

Casters are sold individually. 

Included hardware is a 1/2"-20 x 1-1/4" Long, Grade 8 steel Flange Head bolt with a thin Nyloc nut.