An apparatus for reducing the radius of the bend in a U-shaped tube comprises two squeeze dies and a back die. The back die is hollowed to form a chamber that accommodates the bent section of the tube and supports the entire circumference of the tube at the center of the bent section. Mating slide surfaces on the back die and the squeeze dies cooperate to extend the back die during squeezing in such a manner as to accommodate the longitudinal extension of the tube legs that results from the squeezing; i.e., there is no relative longitudinal movement between the squeeze dies and the tube legs. This arrangement enables relatively thin-walled tubes to be squeezed without undue deformation of the tube.