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Worked on my first disc brake bike yesterday. It was a full-suspension BSO with cable disc brakes.

The rear wheel had two broken spokes, and they where threaded such that I could not replace the spokes without removing the disc (and the cluster). When it came time to remove the disc, five of the screws holding it were standard Allen head screws, but the sixth was a "Torx" style splined head (which required a run out to Home Depot to fetch a set of Torx bits). The screws were all same diameter, length, and thread.

Was there any reason for this one odd screw? I saw no evidence that the disk had been removed previously.

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    Weird; It's not uncommon to have all the bolts be Torx since they're less likely to strip out at the smaller sizes on a rotor, but I've never heard of mixing them. Maybe the guy at Walmart who was putting it together was doing a few bikes at the same time and accidentally mixed the bolts? – Batman Jun 29 '16 at 21:35
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    All the rotors I've seen have used Torx heads. – Kim Ryan Jun 30 '16 at 0:42
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    A BSO? Assembled strangely, you say? How unusual !! – Criggie Jun 30 '16 at 1:25
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    Also possible that the factory/place where the disc was attached had a bin of screws. When one type ran out they dumped a box of the new ones in. You happened to get the cross over bike. Large scale factories in foreign countries often have low or non-existent standards for lower end parts. As long as it works, it works. – Deleted User Jun 30 '16 at 18:08
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    I once saw a BSO in a department store that had one torx that it claimed was an anti theft bolt much like the odd one out on locking car lugs. Could be the case, marketing for dummies. – Nate W Oct 24 '16 at 22:03
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Many of the smaller bolt heads on MTB components tend to be Torx these days, especially for certain brands. For example, of the components I own, the SRAM mtb derailleurs, brake levers, brake rotors, shifters, etc use Torx-headed bolts, yet all the SRAM road components I own are strictly hex keyed. Why they don't stick to one style- preferably Torx- I don't know. I do know that Torx provides more "bite" which results in a bolt head that is less likely to strip, especially in smaller sizes. This makes a lot of sense, especially where higher torque is required such as brake rotor bolts which typically require a higher torque specification in comparison to similarly sized hardware.

That said, there would be no real technical or performance reason to have only one Torx bolt per rotor. I know that many cars with alloy wheels use a single lug nut with a special head per wheel and normal lug nuts on all the rest of the bolts for theft protection. Perhaps this is supposed to be something analogous to that. Alternatively, since you stated that this was a BSO (or Bike-Shaped Object, usually referring to a low quality or department store bike), it's possible that a) a single Torx bolt is used to discourage low skilled workers that are assembling the bikes from removing the rotors, especially if they are of differing sizes, or b) this is simply what they had laying around and the single count on each rotor is a coincidence.

Bottom line, you would not find this as a "feature" of disc brakes or bikes equipped with them of any real quality.

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