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In motorcycles, the exposed mechanical parts play important roles in the styling design. The bolts that fasten these parts a good appearance in addition to the fastening function. Since many parts of a motorcycle are designed so as to appeal the mechanical and metallic beauty, needs are higher for silver-color bolts. Motorcycle bolts are exposed to the external environment, so they are required to have also cosmetic durability. Though there is a surface treatment having a higher cosmetic durability such as the chromium plating, due to its metallic gloss like a mirror, chrome-plated bolts are applicable only to specific models and parts. In this study, we developed a surface treatment that enhance the appearance and cosmetic durability required of bolts for motorcycles. The layer of developed surface treatment is dip coating with thin film polyorganosiloxane material on zinc plating.

A black surface treatment without using hexavalent chromium and applicable to bolts of motorcycles has been developed. The surface treatment also satisfies the requirements of bolts for motorcycles such as fastening performance, corrosion resistance, weather resistance and appearance of outer finish. There is a method of coating a black chemical conversion film using trivalent chromium on the zinc-plated surface as one of the surface treatments that do not use the hexavalent chromium. Such a black film, however, is thinner than the film using hexavalent chromium, and unable to ensure satisfactory black appearance. Meanwhile, although the dip-coating using a black paint can provide a sufficient black color, it is necessary to apply coating two times to eliminate an irregular color. As a result, thick paint films are formed on the roots of thread, making it difficult to fit into the internal thread. To simultaneously solve the above-mentioned two issues, two measures were executed.