a combination of a locknut and key therefor , the locknut including a body portion having a face thereon and a curvilinear groove of predetermined configuration in the face , a thread on the body portion , the thread including a frustoconical lead - in portion terminating at an extremely short cylindrical helical thread portion , a key housing having an internally threaded end portion which can be centered on the lead - in portion and thereafter threaded onto the cylindrical threaded portion of the locknut , the key having a key head with a flange of curvilinear configuration for mating engagement with the groove in the locknut , the key head being mounted on a shank on which the housing is rectilinearly slidable , a spring in the housing for biasing the key head toward the locknut when the housing is centered on the the locknut and which forces the flange into the groove as the housing is threaded onto the locknut , and a wrench - receiving head on the shank for turning the key head after the housing is securely mounted on the locknut .