The Powder Coated Marine Grade Aluminum Rail Base has a 1.0" diameter Rubber Ball designed to sit at right angles to the mounting rail. The Zinc Coated metal U-Bolt and nuts included will fit a circular rail with diameter from 1/2" to 1.0". To mount the rail base, place the top of the base on the rail and then slide the u-bolt around the rail, into the holes in the base top. Now tighten the screws onto the u-bolt until the base is clamped firmly. If you desire, you can now slot the black plactice covers over the exposed ends of the u-bolt. For a larger rail, you can purchase the RAM-B-231LZS Large Rail Base for circular rails with diameter from 1/2" to 1 3/8". The "U" in the part number reflects product packaged in poly bag. For additional specifications, see components and drawings tabs.       MATERIAL:   Powder Coated Marine Grade Aluminum   BALL SIZE:   1 Inch Rubber Ball   COLOR:   Black Powder Coat   WEIGHT:   0.65 lbs     RETAIL:   $31.85