PURPOSE: To reduce machining mandays via the reduction of number of processes, and further reduce the facility cost via the elimination of an expansive tool and a boring machine of high rigidity by deciding an accurate center position for a cylinder block bore surface without fine boring, while eliminating a fine boring process, as a pre-machining process for the fine horning of the cylinder block bore surface

CONSTITUTION: Before the surface is subjected to surface corrosion prevention treatment at a process 15, the bore surface 6b of a high silicon aluminum cylinder block 6 is sequentially subjected to rough boring at a process 12, rough horning by use of an NC horning machine 16 at a process 13 with the position of the support axis 10a of a horning tool 10 kept at the predetermined bore center for the cylinder block 6, and fine horning at a process 14.