Insert core and method for manufacturing a cylinder for internal combustion engine by making use of the insert core

There is provided a method of cast molding a cylinder for an internal combustion engine by making use of an insert core, which makes it possible to rationally form the chamfered portions at the rim portion, on the cylinder bore side, of the scavenging ports, the suction port and the exhaust port with high precision and high freedom in dimension and configuration without fluctuation in size and configuration of the chamfered portion. Herein, the insert core is formed of a cylindrical body having a slightly smaller outer diameter than the diameter of cylinder bore, a scavenging port-forming portion projecting radially outward from the cylindrical body and having almost the same configuration as that of the scavenging port, a suction port-forming portion projecting radially outward from the cylindrical body and having almost the same configuration as that of the cylinder bore-side end portion of suction port, and an exhaust port-forming portion projecting radially outward from the cylindrical body and having almost the same configuration as that of the cylinder bore-side end portion of exhaust port. The insert core further includes chamfered portion-forming portions for forming a chamfered portion at each of the rim portions on the cylinder bore side of the scavenging ports, the suction port, and the exhaust port.