Manufacturer Description The High Strength Plastic Rail Base has a 1.0" diameter Rubber Ball designed to sit at right angles to the mounting rail. The Rail 'U' Clamp will fit a circular rail with diameter from 1.0" to 1.1". To mount to a circular rail, simple press the plastic 'U' clamp around the rail and then connect and tighten the small nut and bolt to close the 'U' clamp. NOTE: Placing a rubber lining (not included) between the tube and the mount for a tube with a diameter less than 1" will help the base 'grab' the rail.