Note: Central Boilerlists its 1"Pex products by their nominal outside diameters (the industry standard method),but lists its 1-1/4" Pex products by their inside diameters.All Central Boiler fittings listed for 1-1/4"Pex, including this product, are actually 1-1/2" Pex fittings.(The MIP side of this fitting is 1-1/4".)If this product will be used with CentralBoiler 1-1/4"ThermoPEX or other Central Boiler branded 1-1/4" Pex line, then it willfit.However, this fitting will not workwith industry-standard 1-1/4" Pex lines but will work with1-1/2"standard Pex lines.