Abstract: For adjustable length linear rods, such as the rods of linear actuators, employing threadedly connected internally threaded female and externally threaded male members, an antirotation lock is provided which includes a pair of semi-circular keys each including a projection fitting in a notch on the end face of the female member and an internal flat cooperating with an external flat on the male member. Each key includes a conical surface cooperating with a conical surface of a ring driven toward the female member by a nut threaded on the male member, thus axially to force the projections into the notches, and radially to force the flats of the keys against the flats of the male member.