A method for inserting an undulated coil assembly in slots of a hollow core of a dynamoelectric machine, in which the slots extend radially inwards, the coil assembly having adjacent superimposed linear portions extending parallel to each other and a plurality of turn portions connecting the linear portions, the method comprising the steps of: - positioning at least a first coil portion of the coil assembly around a support member having a longitudinal axis parallel to the longitudinal axis of the hollow core; - positioning the support member inside the hollow core; - aligning in axial and circumferential directions a guide assembly with respect to the end faces and the slots of the hollow core, said guide assembly having guide surfaces (302,303,304) configured to form a guide passage; - aligning a feed member between the support member and the guide assembly; - relatively rotating the core and the support member in a synchronized way with respect to the guide assembly during the unwinding of the first coil portion from the support member and the feeding of the first coil portion along the guide passage to position the slots for receiving the superimposed linear portions; - unwinding the first coil portion from the support member by rotating said support member along its longitudinal axis; - directing and engaging leading portions of the first coil portion from the support member into the guide passage of the guide assembly by the feed member in such a way to insert the adjacent superimposed linear portions in the slots along a direction of insertion which is radially outwards; - reducing, during the insertion in the slots, the angle of the angular orientation of the adjacent superimposed linear portions with respect to the radiuses of the slots, the reducing step being caused by engaging and moving the superimposed linear portions along the guide passage. - positioning at least a first coil portion of the coil assembly around a support member having a longitudinal axis parallel to the longitudinal axis of the hollow core; - positioning the support member inside the hollow core; - aligning in axial and circumferential directions a guide assembly with respect to the end faces and the slots of the hollow core, said guide assembly having guide surfaces (302,303,304) configured to form a guide passage; - aligning a feed member between the support member and the guide assembly; - relatively rotating the core and the support member in a synchronized way with respect to the guide assembly during the unwinding of the first coil portion from the support member and the feeding of the first coil portion along the guide passage to position the slots for receiving the superimposed linear portions; - unwinding the first coil portion from the support member by rotating said support member along its longitudinal axis; - directing and engaging leading portions of the first coil portion from the support member into the guide passage of the guide assembly by the feed member in such a way to insert the adjacent superimposed linear portions in the slots along a direction of insertion which is radially outwards; - reducing, during the insertion in the slots, the angle of the angular orientation of the adjacent superimposed linear portions with respect to the radiuses of the slots, the reducing step being caused by engaging and moving the superimposed linear portions along the guide passage.