On internet forums, there’s one thing I read that always makes me cringe: people looking to “upgrade” all the bolts on their bike to stainless steel or titanium. If you’re not an engineer, or haven’t worked in a shop, you may not know that all bolts aren’t the same. It’s OK, I’m here to explain.
Almost every bolt on a bike these days is a metric thread, which you probably already knew. Metric steel bolts come in a variety of strength classes. Most common to bikes are 8.8, 10.9, and the strongest, 12.9. Many bolts you buy from the hardware store or a bolt supplier will have the strength class marked on the head. Bolts on your bike though, in an effort to look pretty, may have no such markings.
Before any bike product is for sale, it undergoes a variety of strength and fatigue tests as dictated by the Consumer Product Safety Commission (CPSC), or the International Organization for Standardization (ISO). These are brutal tests designed to push the products above and beyond what they’d see in everyday use. Bolts need to withstand this testing, and thus many structural bolts, like on your stem and seat post, are going to be rated at the 10.9 or 12.9 strength class.
If you’re shopping around for stainless steel, likely you’re going buy 18-8 or A2 bolts. They look nice and won’t rust, but based on material properties they would be rated below the 8.8 strength class. In many cases these fasteners would not pass the required structural testing, which is why all the bolts on your bike aren’t stainless to begin with.
Titanium Bolts are much pricier than steel, but about 40% lighter for the weight weenies out there. Most commonly used for these high end fasteners is Grade 5 Titanium (6AL-4V), which slots in just below the steel 10.9 strength class. Due to this great strength to weight ratio, titanium bolts are often used in high end, super light components where every gram counts. Great for your ultra light road bike, maybe think again on the DH rig.
So back to the original question – is it safe to swap out those rusty bolts with stainless or titanium? Without structurally testing the component with the new fasteners, it’s hard to say. Just because your buddy did it, and he’s still alive, doesn’t always mean it’s safe.
I’d heavily discourage it in any place where a bolt failure could result in a crash (stem, seat post, brake rotors and calipers, etc.) In non critical spots like your water bottle cage, or headset top cap, it doesn’t matter much.
If you’re still going to do it, make sure you’re buying from a reputable source. There’s no way I’d trust my life with some random bits I picked up on Amazon. Even if they claim to be the correct material, you have no way to know for sure. Vendors like McMaster-Carr and Pro-Bolt are much better places for quality product.
One last thing to keep in mind – if you swap bolts and something breaks, it’s unlikely your bike will be covered under a manufacturers warranty. Ride safe!