Hot tub pump union gasket with ribbed o'ring on one side and flat on the other side measures about 3-1/2" outside diameter and is labeled and referred to as a 2-1/2" gasket which fits hot tub pump and heater unions referenced as 2-1/2". Made of a silicone based material for long life.
 
Details:
Brand Waterway
Waterway P/N 711-6020
Outside diameter 3-3/8"
Inside diameter 2-1/2"
Width 1/2"
Flat on one side, o-ring rib on other side
Colour: clear opec
Waterway part # 711-6020