Abstract: An engine block heater adapted for insertion through an opening in the block wall of an internal combustion engine has a novel arrangement for securing the heater within the opening of the block wall. The heater includes a plug member having a heating element and an adjustable fastening assembly extending from one side of the heater into the interior of the engine block. The fastening assembly passes through the plug member for adjustment from outside the block. A spiral spring is further included having its center positioned in fixed relation with the fastening assembly. The spring spirals towards the one side of the plug member. Additionally, two wing members are mounted in pivotal relation with the fastening assembly through an intermediate support member threadably mounted on the fastening assembly. The wing members have an inverted V shape with respect to the one side of the plug member when positioned within the spiral so as to facilitate insertion of the plug member into the aperture.