PURPOSE: To facilitate breakage of a riser part after casting and to prevent detachment of a hollow pipe by separately forming a pipe inserted in the riser from the hollow pipe inserted in a cam shaft part, jointing both pipes with fitting and forming a core material.

CONSTITUTION: At the time of applying hitting to the riser part 13, a casting 10 is broken at a neck part 12, but as the hollow pipe 17 inserted in the riser part 13 is only fitted into the hollow pipe 16 inserted in the cam shaft part 11, the fitting is come out with the hitting applied to the riser part 13. By this method, the breakage of riser part 13 is extremely facilitated and as it can be prevented to apply too much force to the hollow pipe 16 in the cam shaft part 11 at the time of breaking, the detachment of hollow pipe 16 is not developed.