This check seat is designed to fit the 3/4 - 1" Watts Series 909-HW backflow preventers. This seat will fit either the first or second check. The seat is affixed to the check cover with a quarter turn to lock it in place and houses the check poppet and spring. Be sure to lubricate the seat and cover O-Rings before installing. Stainless steel seat is approved for hot water. No special tools are required for servicing. Genuine Watts replacement parts.