METHOD OF MANUFACTURING INTERMEDIATE PRODUCT WITH ENGINE VALVE BOSS PORTION

Provided is a method of manufacturing an engine valve intermediate product such as will cause only a boss portion for formation of a piston portion and/or vane portion to be imparted with the required high strength, and such as will permit the boss portion to be formed at any desired location separated by some distance from a valve head forming portion at a stem forming portion. A method of manufacturing an engine valve intermediate product (2) with a boss portion (11) at which a valve head forming portion (4), a maximum outside diameter of which is greater than that of a stem forming portion (3), is present in integral fashion, and at which the boss portion (11), an outside diameter of which is greater than that of the stem forming portion (3), is formed in integral fashion at an outer circumferential surface at an intermediate location on the stem forming portion, comprising a stem retaining operation in which an intermediate location of the stem forming portion (3) is retained by a plurality of intermediate jigs (6a through 6c) capable of being removably installed radially with respect to a center of the stem forming portion (3), and a base end portion (3b) is retained by a base end jig (5); and a stem enlarging operation in which compressive forces are applied from both ends of the intermediate product (2) and the boss portion (11) is formed as the intermediate product (2) is made to rotate and alternating loads that cause tensile forces and compressive forces to act in repeated and alternating fashion at the outer circumferential surface of the stem forming portion (3) are applied thereto.