Abstract: Apparatus for use in inserting a flexible O-ring into a groove provided in a workpiece, comprising a first piston and cylinder device provided with a hollow piston rod having a slot through which an O-ring is fed by a chute into the hollow piston rod, a plunger which is moved by a second piston to push the O-ring along the hollow piston rod, and radially extending flexible members are provided on the plunger to deform the O-ring as it is moved along the hollow piston rod, the first piston and cylinder device being used to move the hollow piston rod so that its free end lies adjacent to the groove so that when the plunger pushes the O-ring out of the hollow piston rod it enters the groove.