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Production of a relatively inexpensive titanium connecting rod for a mass production motorcycle engine has been achieved. Previously, the high cost of Titanium and other factors limited its usage by the aerospace industry and motor sports racing. As a material for mechanical parts titanium is light in weight and high in strength. In the engines of racing motorcycles the connecting rods, valves, and valve spring retainers which have been fabricated from a titanium alloy enhance the engine's performance.-1, -2 The cost of titanium alloy and its surface treatment is very expensive. -3, -4 Therefore the cost of the material was prohibitive and limited the application of titanium to competition machines. This paper outlines the development of a new surface treatment and application to titanium connecting rods. It was specifically applied to the connecting rods of a 750 cc motorcycle in 1987, the first in this field of industry. -5, -6, -7

In recent years, efforts for development of high-performance and compact automobile engine are being made more actively than ever before. The connecting rod bolt is one of those parts which are very much required to be compact and light, since its size and weight affect the performance of the engine. However, if it is used under high stress, delayed fracture can be caused. Therefore, it is a common practice to keep the stress below 1177MPa. The developed alloy (HB149) is a precipitation hardening alloy steel of low carbon martensite, composed mainly of 0.35C-1.2Cr-1.0Mo-0.3V. With lowered intergranular inclusions by suppressing impurity elements such as phosphorus and sulfur, and with reduced grain boundary embrittlement by tempering it at a high temperature, this steel is excellent in resistance to delayed fracture. Using this steel, we developed a 1372MPa class (Class 14.9*) high strength connecting rod bolt.