Method for production of valve head portion of hollow engine valve and hollow engine valve

A hollow engine valve, comprising:

• a hollow shaft portion sealed at one end;
• and
• a valve head portion of a single-piece material attached to another end of the hollow shaft portion;
• the single-piece material being a material having heat resistance comparable to or better than the heat resistance of a material for the hollow shaft portion;
• the valve head portion comprising a cylindrical barrel section and a diameter-increased section at one end of the barrel section, the diameter-increased section including a bottom section;
• a valve head portion hollow hole opening at a side attached to the hollow shaft portion,
• the valve head portion hollow hole having a first inner surface, at the diameter-increased section, having an inner diameter that increases as the first inner surface extends toward the bottom section, and a second inner surface at the bottom of the valve head portion hollow hole;
• and
• the maximum diameter of the valve head portion hollow hole being larger than a maximum outer diameter of the hollow shaft portion;
• and
• the first inner surface and the second inner surface forming a single continuous curved surface.