The seal seat—also known as the stationary seal—rests inside the pump head and makes a smooth surface that the rotating mechanical seal can ride against, effortlessly ensuring a leak-free seal.
When installing the seal seat you will notice it has a rough side and a smooth side (or a rubber side, in the case of the SQN 20). The smooth side should face against the mechanical seal, allowing it to ride against it and create a smoothly-polished mating surface.
The Seal Seat is part of four pieces that are normally replaced when performing a full Jabsco head rebuild. The other parts are: