A way of improving the fatigue strength of valve springs is to increase the tensile strength of oil tempered wire. However, the more the tensile strength increases, the more coiling operation becomes difficult. Therefore it has been difficult to make a great progress in fatigue strength depending on enhanced tensile strength only. In order that the high fatigue strength coexists with the easiness of coiling, high strength wire with controlled hardness on its surface is developed. Further more, the valve spring is treated by nitriding process at higher temperatures than usual, and the optimized triple hard shot peening is performed. As a result, the developed valve spring seems to exhibit the excellent performance of the world highest level
