PURPOSE: To prevent a spherical head from escaping from a ball seat by making a maximum diameter of a large diameter spherical part of a spherical head larger than the maximum inner diameter of the ball seat.

CONSTITUTION: A head 7 is divided by an intermediate circular step part 7a into a lower large-diameter spherical part 7b and an upper small diameter spherical part 7c. An outer diameter of the small diameter spherical part 7c increases as it goes downward, while the maximum diameter thereof is larger than an outer diameter of a shaft part 6. An outer diameter of the large diameter spherical part 7b increases as it goes upward, while the maximum diameter thereof is larger than that of the small diameter spherical part 7c and the maximum inner diameter of a ball seat 4. The head 7 is prevented from escaping from the ball seat 4 even if the small diameter spherical part 7c of the head 7 is worn.