A method of preparing supported catalysts useful for olefin polymerization is described. The value of the opening angle \u03b1 of the flanks of the external thread of the screw is greater than the value of the opening angle of the flanks of the rollers. The primary oxide comprises cerium or hafnium. Each of the hydroformed members are formed with access openings therein to allow spot-welding electrodes to be positioned against an appropriate wall of the hydroformed member and the adjacent portion of the U-shaped structural shell member.