Spa, hot tub and whirlpool bath heaters with 2"" stainless steel heater tube use these 2"" FBT Split Nut Unions to connect the heater tub to the 2"" MBT tailpiece connectors. Split nut unions are in two halves, screwed together over the heater and threaded onto the tailpiece. Split nut design allows for easy replacement without plumbing. Uses two screws which are included. 2"" FBT threads actually measure 3.0"" outside diameter. 2"" split nut union ring mates with 2"" spa heater tailpieces, such as the HU-2MBT15S. Universal design fits many spa, tub and bath heaters used by spa control and spa manufacturers. Replaces Balboa, Gecko, Waterway and more.