7マーク付六角ボルト - Bolt flat head with 7 mark embossed
Bolts with the number 7 embossed on the head are made of S45C steel which has a representative value of 0.42 to 0.48% carbon. Because it contains more carbon than iron, it has good hardenability, and heat treatment improves mechanical properties and is stronger than iron.It is a material that is not too expensive among carbon steels and is widely distributed in the market. Japanese motorcycles brands use these number 7 a lot, the upset version is more recent than the plain head proposed here. Kawasaki used these in the 60s and 70s but replaced them by the upset version with a dot.
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Zinc is now called trivalent plating treatment which is a modern plating that looks similar to zinc without the chemical hazard and better consistency and corrosion resistance. This is the standard for Japanese manufacture and I recommend it for a perfect restoration.
Black aspect trivalent plating treatment which is a modern plating that looks similar to black zinc without the chemical hazard and better consistency and corrosion resistance. This replaces black zinc of early models and is now standard for Japanese manufacturers.
Yellow chromate treatment is a plating often found on small screws for carburetor or to give a rich aspect. Widely used in the 70s it has been replaced by trivalent zinc because of its environmental hazard.