Systems, methods and motor-generators with preform windings and a molded core are described. The preform windings include a first end, a second end, and a plurality of in-slot portions extending from the first end to the second end. The molded core includes a plurality of slots. Each of the plurality of slots including a respective in-slot portion disposed therein. Each of the plurality of slots includes a wall conforming to an outer profile of the respective in-slot portion.