hookooekoo
Senior Retro Guru
There are various grades of stainless. The main grades for fasteners are A2 and A4 (SAE 304 grade and SAE 316 grade respectively). The A4 grade is often known as Marine Grade, and is slightly more resistant to corrosion than A2 grade.
https://en.wikipedia.org/wiki/SAE_304_stainless_steel
https://en.wikipedia.org/wiki/Marine_grade_stainless
I always use A4 grade, because I don't need many, so the cost difference is very small. Ebay is a good source for small quantities.
In terms of head style, I prefer Button Head for bottle cage bolts, as I think it looks nicer. And in my opinion Button Head doesn't really need a washer, as the head is a slightly larger diameter than a standard socket head bolt. However, be aware that the hex socket will be slightly smaller on a Button Head, so you won't be able to achieve the same torque. For example, an M5 bolt (bottle boss size) usually takes a 4mm allen key in a standard hex socket head style, but will only take a 3mm allen key in standard button head style. If you can find bolts with a Torx socket head then these are superior to bolts with hex socket heads, as the Torx socket is purposefully designed not to round out or slip. Note that these are general rules based on standard bolts, so you may find exceptions, such as an M5 bolt that takes a 5mm allen key. For example, Shimano seem to use M5 bolts that take a 5mm allen key for the cable clamp bolts on their dérailleurs.
As an aside, I think it's best to fit a well-greased bolt in all bosses and eyelets, whether you are using them for a rack, mudguards, bottle cage, or nothing at all. Having a well-greased bolt in place helps to stop the thread corroding when you ride in the wet, and keeps mud and grit out.
https://en.wikipedia.org/wiki/SAE_304_stainless_steel
https://en.wikipedia.org/wiki/Marine_grade_stainless
I always use A4 grade, because I don't need many, so the cost difference is very small. Ebay is a good source for small quantities.
In terms of head style, I prefer Button Head for bottle cage bolts, as I think it looks nicer. And in my opinion Button Head doesn't really need a washer, as the head is a slightly larger diameter than a standard socket head bolt. However, be aware that the hex socket will be slightly smaller on a Button Head, so you won't be able to achieve the same torque. For example, an M5 bolt (bottle boss size) usually takes a 4mm allen key in a standard hex socket head style, but will only take a 3mm allen key in standard button head style. If you can find bolts with a Torx socket head then these are superior to bolts with hex socket heads, as the Torx socket is purposefully designed not to round out or slip. Note that these are general rules based on standard bolts, so you may find exceptions, such as an M5 bolt that takes a 5mm allen key. For example, Shimano seem to use M5 bolts that take a 5mm allen key for the cable clamp bolts on their dérailleurs.
As an aside, I think it's best to fit a well-greased bolt in all bosses and eyelets, whether you are using them for a rack, mudguards, bottle cage, or nothing at all. Having a well-greased bolt in place helps to stop the thread corroding when you ride in the wet, and keeps mud and grit out.
