Abstract: In the process of assembling long, large diameter cylinders by welding together two or more relatively short cylinders of the same diameter, apparatus for facilitating the butt welding of the two cylinders comprises, a substantially horizontal boom which is supported by a horizontal trolley attached to a first vertical wall. Two circular cylindrical shells are rolled from sheets of the same width and to substantially the same diameter. They are supported on a plurality of pairs of rollers, which are adapted to coaxially support the two cylinders end to end. The boom includes at its outer end a hydraulic cylinder and piston, with the piston carrying a face plate, with the face concave and of the same radius of curvature as the cylinders. There is a second support wall on the side of the cylinders opposite to the boom, so that a force can be exerted by the piston against the cylinders, to reduce the horizontal diameter of one so as to match that of the other.