HU25RMG Spa & Bath Heater Ridged Molded Gasket to seal spa heater to PVC tailpiece connectors. 1 per pack. Measures 3.4" outside diameter x 2.5" inside diam. Flat on one side facing heater, raised ridge 2.98 od x 2.73 id on opposite side fits in o-ring groove of PVC tailpiece. Sold as 1 per pack. Shipped in clear translucent material. RMG heater gaskets, not for standard sized 2 inch flo-thru heaters this is for big 2.5" diameter heaters or pump intakes where the OD of the pump threads measure 3.7"