| 摘要 |
PROBLEM TO BE SOLVED: To improve strength of an umbrella part and wear resistance of a valve face, by miniaturizing a crystal grain diameter in the umbrella part of the engine valve. SOLUTION: After forming an intermediate work piece 12 with an enlarged diameter part 12b above its stem part 12a by not extrusion forging of a cylindrical material, the enlarged diameter part 12b of the intermediate work piece 12 is hot forged into an umbrella shape, which is the original form almost identical to that of an engine valve to be manufactured. In this forming process of the engine valve, the diameter of the enlarged part 12b of the intermediate work piece 12 is extrusion forged so as to be a little smaller than that of the cylindrical material.
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