Outboard-motor cylinder block and manufacturing method therefor

A method of manufacturing an outboard-motor cylinder block, the outboard-motor cylinder block having, on an outer peripheral surface thereof, a plurality of screw holes for fastening the outboard-motor cylinder block and a member to be fastened, the method comprising:

• casting a cylinder block body using an aluminum alloy as a base material, the cylinder block body having a cylinder bore and a water jacket around the cylinder bore formed on a joining surface to be joined to a cylinder head, and having a provisional hole smaller in an inner diameter than the screw hole at a place where the screw hole is planned to be formed;
• forming an anodized film on a surface of the cylinder block body where the aluminum alloy is exposed, by carrying out an anodizing treatment;
• sealing pores of the anodized film to form a sealed anodized film by carrying out a sealing treatment;
• threading the provisional hole to form the screw hole by removing the anodized film on an inner circumferential surface of the provisional hole such that the aluminum alloy is exposed to an inner circumferential surface of the screw hole;
• and
• forming a chemical conversion coating film on a surface of the sealed anodized film and a surface of the aluminum alloy exposed to the inner circumferential surface of the screw hole.