a process for forming oversized circular pipe has the steps of affixing an elliptically - shaped forming head onto a spiral pipe forming machine , driving a metal strip along the forming head such that the metal strip takes on a shape of the forming head , moving said metal strip out of the forming head into a spiral pattern such that edges of the metal strip engage with each other so as to form an elliptically - shaped spiral pipe , removing the spiral pipe from the forming machine , and forming the elliptically - shaped pipe into a circular shape . the circular pipe has a diameter of greater then forty - eight inches . the forming head has an adjustable diameter .