Abstract: A nut lock device for use in an engine valve train of the type wherein a rocker arm is pivotable about a fulcrum member to effect opening and closing movement of a spring biased poppet valve, the fulcrum member being supported on a support stud and retained thereon by a retainer nut threaded on the support stud, the improvement wherein the bearing surface of the retainer nut is inclined at an angle to a plane extending at right angles to the axis of the support stud and, wherein the nut reaction surface on the fulcrum member is inclined in a complementary manner to that of the retainer nut for mating engagement therewith whereby to define a cam lock preventing rotation of the retainer nut relative to the support stud during engine operation.