Note: Central Boiler lists 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 Central Boiler 1-1/4" ThermoPEX or other Central Boiler branded 1-1/4" Pex line, then it will fit. However, this fitting will not work with industry-standard 1-1/4" Pex lines but will work with 1-1/2" standard Pex lines.