2 Buttress Threaded x 2 Slip Tailpiece Flat 417-5040.This is a flat tailpiece without a groove for an o-ring.
This tailpiece type uses a flat gasket (sold separately).
This is a heater tailpiece with a slip connection in back for either 2 inch inner diameter flex hose or pipe, or the spigot connection from another 2″ PVC part.
Need a flat gasket? See 711-4010.
Need a tailpiece with a gasket/o-ring grove? See 417-5050
