An improved window sash balance and components thereof. The balance includes a spring fixed with respect to the frame and a pair of pulley blocks, each containing one or more pulleys. One block is fixed with respect to the frame and the other is disposed on the end of the spring. A cord is secured to one pulley block and trained over the pulleys. The other end of the cord terminates in a hook adapted to be received in an opening in the window jamb channel. The pulley block unit includes a pulley-receiving frame, one, two or three pulleys received therein, and at least one axle for rotatably supporting the pulleys within the frame. The block has one or two pulley-receiving openings therein, and suitable openings for reception and positioning of the axle. The block includes first and second pairs of assembly reference surfaces forming a part of the block and spaced radially from the axle opening. The axle-receiving opening in the pulley and in the block each include outwardly tapering guide or pilot surfaces and the axle includes one or two inwardly tapering pilot surfaces. The size of the pulley and the positioning of the reference surfaces are arranged so that when an outer edge of the pulley is in coplanar alignment with the reference surface, the tapering margins on the axle and the pulley will engage and serve to align the pulley as the axles are positioned in the block.