Apparatus for thermal sizing elongated, tubular channels of a square cross section includes a die having plural elongated die elements and an elongated mandrel having a frame mounting plural pairs of rollers distributed along its length on all four sides. The die is inserted in the channel with its die elements disposed adjacent the channel corners. The mandrel is then inserted with its roller pairs peripherally engaging the die elements, pressing them against the channel corners. The channel, die and mandrel are then heated to a hot-forming temperature, thermally expanding the mandrel frame into pressing engagement with the die elements in lieu of the rollers to plastically reform the channel to a stable shape free of geometric irregularities.