The invention relates to a closure with a thermal safety function for a hydrodynamic or hydraulic machine, in particular a hydrodynamic clutch, designed for use in a housing of the hydrodynamic or hydraulic machine which holds a working medium, wherein the closure has a closure body for screwing or inserting into the housing or mounting on the housing in some other way; the closure body has a passage into which a closure core is introduced, which closure core is connected, by means of a melting element which can be melted on at a predefined temperature, directly to the closure body or to an intermediate element introduced in a seal-forming fashion in the closure body, in such a way that the passage is sealed off so as to prevent the working medium from passing through. The invention is characterized in such a way that the closure core has a blind bore or a blind hole which is open with respect to the working space, with the result that working medium can flow into the closure core.