Abstract: The object of this invention is to provide the method of altering the size of tetrafluoroethylene tubing which is only available in limited combination of wall thicknesses and diameter. The method includes the steps of sliding the tetrafluoroethylene tubing (12) onto an aluminum mandrel (20) and clamping the ends of the tubing to the mandrel by means of clamps (24). The tetrafluoroethylene tubing and mandrel are then placed in a supporting coil (25). The supporting coil with the mandrel and tetrafluoroethylene tubing are then positioned in a insulated steel pipe (28). The steel pipe is normally covered with a fiber glass insulator to smooth out temperature distribution therein. The entire structure is then placed in an oven which heats the tetrafluorethylene tubing which is then shrunk by the heat to the outer dimension of the aluminum mandrel. After cooling the aluminum mandrel is removed from the newly sized tetrafluoroethylene tubing by a conventional chemical milling process.