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Most bolts on bikes are using hex sockets. However, there is at least one exception: the 6 bolts of brake discs, which usually have Torx sockets.

What is the reason for this? Has the Torx socket a significant importance in this area in particular?

Related question: I'm considering replacing these bolts with titanium version (saves 6 grams per wheel, woohoo!), and I can buy them with either hex or Torx sockets (hex would be a bit more convenient as I have hex keys readily available everywhere, on the other hand, the Torx socket is technically better). Can I just pick the socket I prefer, or would you strongly advise for Torx?

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  • Refs this related question I just found out: Is there anything special about rotor bolts? Commented May 9, 2020 at 3:33
  • There may be warranty and liability issues because the brake was designed like that and it is a safety relevant item.
    – Carel
    Commented May 9, 2020 at 7:25
  • That related question was me. I haven't had any issues with my stainless socket hex head bolts (though I have had other brake trouble). I've also never needed to remove or tighten a rotor on the road and I have plenty of Torx tools at home
    – Chris H
    Commented May 9, 2020 at 13:53
  • Because everyone already has Allen keys. Commented May 9, 2020 at 19:23

2 Answers 2

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The magic of lobes allows Torx to handle a given torque value with a shorter bolt head than the alternatives, and some frame/fork designs need the clearance in this area. That's the only reason for it.

Some secondary bolt retention designs won't work with just any bolts, ie Shimano.

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  • Could we say that, basically, the Torx socket is here to help preventing the key from ripping off because of the small button heads? Commented May 10, 2020 at 1:38
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    Yes. Or in other words, the M5 Allen option for a similarly low-profile bolt head would be a button head screw with a 3mm Allen fitting. That would be borderline for the ~6Nm torque needed. Commented May 10, 2020 at 1:46
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Torx bolts does not need protruded or inset heads like hex screws, and they provide exceptional retention force for the tool when (and if) fitted correctly, hence you cannot strip their heads easily, like in Posidriv or Phillips screws.

They are a good replacement for Philips head screws. Also, not to mention, they are lighter and more streamlined.

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  • I do not understand the first sentence. Perhaps you mean one of these shapes? en.wikipedia.org/wiki/Screw#Screw_head_shapes And I do not understand why that should be a difference between hex and torx. After all, the physics is almost the same, although considerably different from Phillips, which are prone to slip at higher torques. But hex is very comparable to torx in this regard. Commented Dec 7, 2022 at 15:15
  • More force should be applied before a Torx rips, compared to an hex, so the head can be less high (i.e. tool inserted less deep) while being able to sustain the required torque without ripping off. By the way, on previous practices I thought Torx was more ripping than hex… then I bought a proper quality key, and understood Torx hasn't much tolerance to keys that are approximately shaped… but with the proper tool, the benefits are clear. Commented Dec 8, 2022 at 2:53

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