The paper presents the results of an experiment demonstrating that lead-containing machinable Al Cu4PbMg(Mn) can be successfully replaced by a lead-free alternative such as Al CuMgSnBi alloys. However, tin in the Al-Cu-Mg system changes the supersaturated solid solution decomposition sequence suppressing pre-precipitation stages. These changes must be respected in the thermo-mechanical treatment of Al CuMgSnBi machinable alloys. The machinability in turning tests of the new alloys, evaluated through the chip shape and fragility, is in close connection with magnesium content.