For 2" heater unions, this flat gasket has a raised o-ring in the center.
2" connections are the most common heater size, and will work for most Gecko, Balboa, Hydroquip, Waterway, CTI (Correct-Tech) and other spa heaters.
The raised o-ring portion measures approximately 2-3/8" inside diameter and 2-5/8" outside diameter.
The flat gasket portion measures approximately 2-1/16" inside diameter and 2-15/16" outside diameter.
2" heater union tailpieces are here.