Waterway industry standard 2 inch o-ring gaskets part# 711-4030, designed to work with 2 inch flo-thru heater housings, with split nut unions and 1 piece plastic unions. You are getting a pair of gaskets, one for each heater side. This item is called a gasket o-ring, because, it is a gasket and o-ring built into one piece. This helps to assure a tight drip free seal every time. Measurements Inner Diameter: 2-1/8" Outer Diameter: 2-15/16"