thanks for your explanation. Should we stick to stainless or titanium? What do you recommend? I will soon change all the suspension bolts in my car and I want to make sure I use the best quality.
Lotus specified property class 10.9 bolts in this particular application and that should be sufficient, I think the key is using quality hardware with the correct corrosion protection. I have not been able to find if Lotus specified a specific corrosion protection treatment for their hardware.
I would stay away from stainless steel for this particular application due to their lack of strength, I believe they are similar in strength to property class 8.8 metric and Grade 5 standard bolts. Titanium hardware is typically very specialized to specific applications and I would not recommend using it unless an engineer with specific knowledge has recommended it for an application since titanium doesn't play well with many types of dissimilar metals.
Black-oxide alloy steel bolts should be avoided due to their lack of corrosion protection. Zinc-plated bolts provide good corrosion protection in wet environments and zinc-flake-coated hardware provides about 20 times more protection than zinc-plated bolts, and zinc-aluminum-coated hardware is about 40 times as resistant to corrosion than standard zinc-plating. The zinc-flaked-coated and zinc-aluminum-coated hardware can be hard to find in certain sizes and grades.
When I sourced hardware for mine I went with a zinc-plated property class 12.9 bolt, mainly due to availability. I would have used zinc-plated property class 10.9 bolts if I could have found a quality source that didn't require large quantity purchases or long lead times. The plating process, if not done properly, can make embrittlement more likely, and higher grade bolts are inherently more susceptible to embrittlement due to environmental factors, so I plan on re-inspecting the bolts every 2 or 3 years.
Sorry for such a long-winded anorak response, I hope this explanation didn't frustrate or confuse you. I just wanted to put some information out there for others to see or comment on.
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