Securement of flat heat exchange surfaces, particularly fins to tubular conduits of a heat exchanger. The heat exchange material is formed into upper and lower, or first and second pieces having longitudinal, partially circular recesses parallel to the length of a tube for engaging opposite sides of the tube. The radii of the recesses corresponds to the outer diameter of the tube. The recesses extend through less than 180.degree. so that the flat parts of the heat exchange surface are spaced leaving a gap prior to securement to the tubes. The two pieces are initially formed to interengage in such a way that they can be joined by lock seaming on opposite sides of the tube. Forces are applied to the interengaged pieces, forcing them together, stretching the heat exchange material at the recesses, thereby producing a tight interference fit between the heat exchange surfaces and the tube to provide minimum resistance to conduction of heat and at the same time securing the pieces together by lock seaming. The tube remains round and undeformed so the cross-sectional heat exchange area remains the same.