Abstract: A thermally sensitive actuator consists of a pressure cylinder comprising a tube, two end closure elements each secured to a respective end of the tube, a body of flowable material contained within the tube between the end closure elements, and a piston extending through a through-bore in one of the end closure elements and projecting into the body of flowable material. During assembly, the said body of flowable material is disposed within the tube between and abutting said end closure elements, while at least one of said end closure elements is not secured to the tube, so that said body of flowable material determines the distance apart of said end closure elements. The end closure elements are secured to the tube by displacing the peripheral edges of the ends of the latter inwardly.