Abstract: A hydraulic radial piston power transmitting assembly including a hoursing and a back plate with a fixed pintle attached to the back plate which rotatably supports a cylinder block with radially spaced bores and pistons therearound, a drive shaft which drives the cylinder block through a coupling means, a cam ring surrounding the cylinder block which is pivotally connected to the housing allowing limited movement of the ring to vary the piston stroke in the cylinder block and cam ring positioning means including a rotating control arm mounted in the housing, swivel means attached to the cam ring with a spherical surface thereon and a sleeve means slidably received on the control arm means including a lateral bore for sliding receipt of the spherical surface of the swivel means to accommodate the locus of points which define the cam ring's various displacement positions.