I recently needed to replace the rusted bolts on my stem so I went down to Home Depot and purchased some steel bolts that matched what I originally had. After looking at https://www.boltdepot.com/fastener-in...Materials.aspx I learned that not all bolts are created equal. The bolts I purchased had '12.9' stamped on the head, which indicates to me that its class 12.9 Alloy Steel, which should be very strong but brittle according to boltdepot. I have also seen mention on this board of people using stainless steel bolts, which are weaker but won't rust. FSA sells CrMo stem bolts and of course expensive Ti bolts are also available. My question is, what is the appropriate bolt material for stems and is it different for say seat posts or the top cap bolt?

thank you,
eric

12.9 steel is great for everything bike-related. I'm not a fan of stainless because it is weaker, and while titanium is high-zoot, it's unreasonably expensive and on the delicate side for my taste.

Most of the stainless bolts you'll find at Home Depot are plenty strong for a stem. The bolts will pull out the aluminum threads long before breaking and if you're tightening them up enough to break than that's waaaaay too much torque.

I can't think of a bicycle application where the standard Grade 2 steel or typical stainless steel bolt wouldn't be more than strong enough. Possibly square taper crank fixing bolts justify a higher strength rating but nothing else. As Crankycrank noted, the threads in the Al wedge will fail long before the bolt breaks.

FYI, I was informed by Thomson to not use Titanium bolts anywere on a stem.

The bolts that you have is tronger than needed, but certainly fine. A decent quality stainless steel bolt will easily handle far more torque than ever recommended for a bike application and won't rust. I'm a fan of stainless steel bolts for those reasons.

I've bought boxes of 50 M5 bolts for for $5-7 from McMaster-Carr, so I have bolts to replace anything I might need.