This 2.5 inch Rubber Coned End cap is designed to mount on either end of a 12 inch Black Rubber Straight Roller 5/8 inner diameter. These end caps provide extra protection for your boat hull. They typically mount on the outside of a roller bracket on the same shaft as the straight roller. Each Rubber End Cap would take up 1 1/2 inches of the shaft. These rollers and end caps are common mounted on a 12 inch Galvanized Roller Bracket with the appropriate roller shaft length. In most cases using this product in addition to the 12 inch straight roller requires using a slightly longer shaft than the 13.5 inch roller shaft.