INTERNAL-COMBUSTION-ENGINE PISTON AND PIN-HOLE-FORMING METHOD THEREFOR

A piston of this invention is cast 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.