Method for manufacturing cylinder block and cylinder block

A method of manufacturing a semi-closed deck cylinder block, the semi-closed deck cylinder block including a cylinder wall of a cylinder into which a piston is to be inserted, an outer wall that surrounds the cylinder wall with a water jacket interposed therebetween, and a bridge that connects the cylinder wall and the outer wall to each other and blocks a part of an opening of the water jacket at a top deck of the cylinder block, the method comprising the step of:

• pressing a probe of a friction stir welding tool against a central part of an upper surface of a bridge member installed in the opening of the water jacket at the top deck, the probe rotating about an axis parallel to a cylinder axis of the cylinder;
• keeping the probe pressed against the central part for a predetermined time to cause side surfaces of the bridge member to expand as a result of the probe being pressed against the upper surface thereof and to come into contact with both the cylinder wall and the outer wall;
• after the predetermined time, moving the probe to the cylinder wall or the outer wall while keeping the probe pressed against the upper surface to friction-stir weld the bridge member with the cylinder wall or the outer wall to which the probe has moved;
• and
• removing the probe from the top deck after the welding step.