Abstract: An arrangement for working the interior cross-section of a thin-walled circular pipe or other cylinder to a symmetrical, non-circular cross-section such as a rectangular or a flat-oval form is described. The pipe to be worked is advanced over and into contact with a conforming mandrel whose outer surface has a cross-section that corresponds to the internal, initially round cross-section of the pipe. The mandrel is provided with an outer cross-section that varies axially from an input round cross-section corresponding to that of the pipe to an output cross-section that matches the final configuration of the worked pipe, while exhibiting a constant peripheral dimension throughout its length. The mandrel is made freely-rotatable about its axis as the pipe is advanced along and in contact with the mandrel in order to maintain a coaxial configuration of the resulting contoured pipe.