Abstract: A hand tool for installing retainer rings includes a frame element, a pair of swing arms and a threaded drive shaft. The frame is an axially elongated post element having a pair of pivot joints symmetrically disposed on opposite sides of a post axis along a line substantially normal to the post axis. A pair of swing arms attached to the pivot joints for partial rotation about respective pivot joint axes, extend together along the post axes away from a handle portion of the post length. Retainer ring engagement nibs are secured to extension arms that project from distal ends of the swing arms. Threaded follower blocks pivotally secured to each of the swing arms receive respectively threaded opposite ends of the drive shaft. A midposition of the drive shaft is secured to the post element with rotational freedom and axial restraint. Thread leads of the two follower blocks are oppositely turned to produce oppositely directed movement of the retainer ring nibs when the drive shaft is rotated.