Methods of assembling rotors having non complementary flange openings

Methods for assembling a first rotor and a second rotor having non-complementary flange openings may include providing mating elements at substantially the center on corresponding rotor flanges, coupling dowels into each of a plurality of circumferentially spaced flange openings in one rotor flange, and mating the mating elements and extending the dowels through corresponding flange openings in the other rotor flange. The rotors are then progressively drawn closer by threading a threaded collar onto each dowel while ensuring alignment between the rotor flanges. Once the flanges contact, machining the flange openings makes them complementary. Once the dowels are removed, the rotors can be coupled by a fastener in each set of complementary flange openings.