This check seat fits the 4" Watts 909 Series reduced pressure detector assembly (RPDA) backflow preventers. This seat will fit either the first or second check assembly. To remove the old seat, slide two retainer wires out from under seat and pull seat straight up. Be sure to lube the O-Ring before installing new seat. Does NOT include parts for the bypass assembly. No special tools are required for servicing. Genuine Watts replacement parts.