Pentair WC78-38T 3/4-Inch NPT Pipe Plug Replacement for Sta-Rite Posi-Flo II Pool and Spa Cartridge Filters Pipe plug replacement Fits with Sta-Rite Posi-Flo II pool and spa cartridge filters model PTM50, PTM70, PTM100 and also compatible with PTM135 Series Filter pipe plugs have 3/4-inch NPT threads