| 摘要 |
PROBLEM TO BE SOLVED: To provide a forged piston for four-cycle engine to increase output and reduce weight, while evading problems such as seizures or scuffing due to increase of surface pressure, and to extend life by improving seizure resistance. SOLUTION: This forged piston for four-cycle engine is equipped with a generally disk-like head portion 2, a skirt portion extending downward from at least both sides with respect to the piston pin axis of the outer peripheral edge of the head portion 2, at least two ring grooves 2d, 2e formed on the side peripheral surface of the head portion 2, and an oil hole that is formed in the bottom of the lowest ring groove 2e interconnecting inside of the piston. The piston 1 is manufactured by closed die forging with aluminum alloy containing at least silicone. Fiber flows f are formed on a skirt portion 3 extending in the sliding direction. Between the fiber flows f on the sliding surface of the skirt portion 3, oil retainer grooves 7a are formed by shot peening.
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