Abstract: A pin bore is formed on a piston of the internal combustion engine, wherein the piston is connected with a connecting rod through a piston pin, and includes pin bosses through which the pin bore is formed for receiving the piston pin. The piston is casted from aluminum silicon alloy. At least the piston center side portion of the pin bore is formed from the taper hole such that the piston center side is large. Dimple process is applied on the inner surface of the pin bore, wherein the recesses by the dimple process are a lubrication oil reservoir. Also, the first phase silicon crystal in the piston matrix is minimized by the dimple process for forming the minimized layer on the inner surface of the pin bore.

Abstract: A piston of this invention is casted from aluminum silicon alloy, a piston center side portion and the piston peripheral side portion of the pin bore 20 formed on the pin boss 18 are formed with the tapers 27, 28, and recesses 33 are formed on the inner surface of the pin bore 20 excepting the area near the engaging ring grooves 24, through the dimple process, and the first phase silicon crystal in the piston matrix is minimized by the dimple process and the inner surface of the pin bore 20 is reinforced by the minimized layer.