A preformed frame for a harvesting machine header having a main beam that is constructed of a single piece tube extending from end to end of the header. The single piece tube is preformed by vertically cutting the tube at a location intermediate the two ends and bending the tube to raise the ends upward and forward. A cap covers the top, front and rear walls of the beam and a gusset plate is placed on the bottom of the single piece tube at the location of the cut. The gusset plate, cap and single piece tube are welded together to hold the preformed shaped of the tube. When the header is supported above ground, the deflection brings the cutterbar, not the main beam, to a substantially straight and horizontal position.