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Rather than replace the stem on every rusty bike I come across, I want to replace some stem bolts which are rusted, cheap original ones. I know they can differ in size a bit so I would get some that fit nicely the profile of the existing stem.

Are there any specific requirements for bolts to do this job? Can I just bulk buy some stainless steel socket head cap screws?

In particular this is for other people's bikes, or refurbishing ones to be sold. I would maybe try things out on my own bike but don't want to take any risk with someone else's.

Most jobs on the bike I would be more confident about subbing bolts in, or they usually have better lasting bolts, but stem bolts seem really safety critical to me but often rust and deserve replacing. Am I worrying where there is no need?

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    The top bolt is not critical at all because it is only used for setting the free play of the head-set and doesn't bear any load after that. The pinch bolts are normally tightened to a max of 3-5Nm, same for those holding the bar. Quality stainless bolts will perfectly do the job. Quality stems use stainless bolts btw.
    – Carel
    Commented Apr 10, 2020 at 9:10
  • @Carel aye, pinch bolts and plate bolts are the issue. Prompts the question, how do we judge quality of bolt? Maybe that is the question
    – Swifty
    Commented Apr 10, 2020 at 9:30
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    I've prettified bolts like this, by soaking in rust converter, a brief touchup with sandpaper, and then a light spray of black rust blocker paint. Let the paint harden for a couple days before refitting to prevent damage. Apparently "evaporust" does similar but I've not used it - I used a phosphoric-based converter that changes brown rust into black phosphoric oxide which doesn't progress.
    – Criggie ♦
    Commented Apr 10, 2020 at 11:46
  • @swifty :Stainless steel is more tension resistant than standard (8.8) steel that is normally used for bolts.
    – Carel
    Commented Apr 10, 2020 at 17:54
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    It's a good question. There probably is a right technical answer out there in terms of what performs best in ultimate terms. In BMX land I know at least some stems come with grade 8 socket cap screws, which while not stainless are pretty corrosion resistant in their black oxide coated form, and hardware stores tend to always have them, so that's one thing I think you can grab and feel good about using.
    – Nathan Knutson
    Commented Apr 11, 2020 at 2:27

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Most bolts should have their tensile range marked on them somewhere, often stamped inside the hex cap. There should be a short number - 3.6, 8.8 etc.. Even a fastening rated as low as 3.6 is capable of 27 tons per square inch, more than 300n per square meter. A stainless steel M6 can probably handle 1.25 tonnes, the aluminium you're bolting into / through will fail before that.

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Stainless steel bolts are considerably weaker then steel ones. But, it's still good enough to the job.

One reason I don't use stainless steel bolts is, they are more reactive with aluminium, then mild or hardened steel, so promote galvanic corrosion even more.

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  • The first sentence is strange. Stainless steel is a particular class of steels (many various ones), but is also steel.
    – Vladimir F Героям слава
    Commented Dec 7, 2022 at 15:07
  • But also includes nickel and chromium...
    – Barış Atasoy
    Commented Dec 8, 2022 at 9:30
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    So do some non-stainless steels. And in considerable amounts. A steel is in practice hardly just a binary compound of iron and carbon.
    – Vladimir F Героям слава
    Commented Dec 8, 2022 at 9:41

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