An electric plug-in connector, comprising a plug head on which a union nut is rotatably disposed for screwing the plug-in connector together with a mating connector, can be screwed together with the mating connector under adherence to a prescribed tightening torque even without a torque wrench if the union nut consists of a threaded sleeve with a ring which comes into engagement with the threaded sleeve by way of driving parts at least during screwing on, of which the one driving part is dimensioned in such a way that it is shorn off by the other driving part when exceeding a tightening torque.