Swimables SW-41-120 Manifold O-ring Kit for MasterTemp Max-E-Therm Heater - This Manifold O-ring Kit is for use with the Pentair MasterTemp Max-E-Therm Heater. It includes 1 rectangular manifold O-ring and 12 small O-rings. An O-ring is a round mechanical gasket used in static or dynamic applications. O-rings block paths that allow liquid or gas to escape.