Abstract: A block heater for the cooling fluid of an engine block has a main body with a surrounding flange at the front face for engaging into an aperture in the block. A clamping element attached to the rear face can be actuated by a screw which is rotated from the front face to pull the clamping element into engagement with the inside surface of the block to cooperate with the flange in a clamping action. The clamping member cooperates with the screw to be pulled forwardly. The rear face of the clamping member has a countersink around the threaded bore therethrough and the screw has a bore in the end face thereof so that a portion of the screw material can be worked into a flange surrounding the end of the screw to engage into the countersink portion. This arrangement prevents reverse rotation of the screw from releasing the clamping member for discharge into the engine compartment.