A decoupler assembly for transferring torque between a shaft and an endless drive member, said decoupler assembly comprising: a hub that is adapted to be coupled to the shaft such that the shaft co-rotates with the hub about a rotational axis; a pulley rotatably coupled to the hub, the pulley having an outer periphery that is adapted to engage the endless drive member; a helical torsion spring concentric with the rotational axis and having a first axial face and a second axial face, and having a plurality of coils which are spaced apart by a plurality of gaps; a first engagement structure positioned between the torsion spring and one of the hub and the pulley, wherein the first engagement structure includes a helical first axial shoulder for engaging the first axial face of the torsion spring; and a second engagement structure positioned between the torsion spring and the other of the hub and the pulley, wherein the second engagement structure includes a second axial shoulder engageable with the second axial face of the torsion spring, wherein rotation of the pulley in a first rotational direction relative to the hub drives rotation of the hub through the torsion spring, and wherein rotation of the hub in the first direction relative to the pulley generates relative rotation between the torsion spring and the helical first axial shoulder which causes axial compression of the torsion spring between the first and second axial shoulders (51, 67), wherein the plurality of gaps are sized to provide a selected amount of axial compression of the torsion spring, characterized in that there is a selected finite amount of relative rotation available between the hub and the pulley prior to lock up of the spring due to elimination of the gaps from axial compression. a hub that is adapted to be coupled to the shaft such that the shaft co-rotates with the hub about a rotational axis; a pulley rotatably coupled to the hub, the pulley having an outer periphery that is adapted to engage the endless drive member; a helical torsion spring concentric with the rotational axis and having a first axial face and a second axial face, and having a plurality of coils which are spaced apart by a plurality of gaps; a first engagement structure positioned between the torsion spring and one of the hub and the pulley, wherein the first engagement structure includes a helical first axial shoulder for engaging the first axial face of the torsion spring; and a second engagement structure positioned between the torsion spring and the other of the hub and the pulley, wherein the second engagement structure includes a second axial shoulder engageable with the second axial face of the torsion spring, wherein rotation of the pulley in a first rotational direction relative to the hub drives rotation of the hub through the torsion spring, and wherein rotation of the hub in the first direction relative to the pulley generates relative rotation between the torsion spring and the helical first axial shoulder which causes axial compression of the torsion spring between the first and second axial shoulders (51, 67), wherein the plurality of gaps are sized to provide a selected amount of axial compression of the torsion spring, characterized in that there is a selected finite amount of relative rotation available between the hub and the pulley prior to lock up of the spring due to elimination of the gaps from axial compression.