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.
Two types of treatments are proposed:
- Trivalent plating treatment 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 recommand it for a perfect looking and lasting restoration.
- Chromate plating for resistance to corrosion. Electrogalvanizing is applied to the base, and then a chromate treatment using hexavalent chromium, which is a chemical conversion treatment, is applied. The chromate film changes its color from blue to yellow to red to red-green to green-brown to brownish brown as the chromate film immersion time of the zinc plating solution increases and the thickness of the chromate film increases. Hexavalent chromium is subject to RoHS regulations but still widely used. These are the few yellow bolts on our bikes.
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