An antirunaway device for a hand grenade fuze utilizes a spring biased deted firing pin-slider assembly to interlock a torque biased rotatably held detonator assembly in an out-of-line "safe" position when the biased firing pin-slider assembly senses the absence of a restraining, properly functioning, axially aligned dashpot timer assembly. A slider blocking member is depressed into a groove provided therefor, by the slider assembly, to allow the detonator assembly to rotate into an in-line "armed" position when the slider assembly mechanically senses the presence of a properly functioning dashpot timer assembly, but remains in a blocking position when the timer is not properly functioning.