Waterway industry standard 1.5 inch o-ring gasket part# 711-4050. They are designed to work with 1.5" flo-thru heater housings, 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 insure a tight drip free seal every time. Measurements Inner Diameter: 1-7/16" Outer Diameter: 2-5/16" Thickness: 1/8"