Abstract: A mechanically deformable heat transfer fin for installation in a hollow cavity of a large complex elongated extrusion to improve heat transfer characteristics without brazing welding or glueing. Since the required design cannot be obtained with the transfer fins as an integral part of the extrusion using presently known techniques, a separate fin insert of slightly smaller dimensions than the hollow opening of the large extrusion is installed therein. The insert includes a plurality of legs and an internal pressure cavity which can be sealed off at both ends with pressure cap fittings secured by tie rods. After installation in the extrusion, pressure is applied to the inner chamber until the legs of the insert move outward and contact the inner surface of the extrusion walls. The pressure is raised until the relatively thin walls of the insert are permanently deformed so that, when the pressure is relieved, the insert is held fixedly in place.