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While trying to replace the yoke of my derailleur I made the mistake of using a multi-tool hex key that was not able to handle the torque necessary for the task and snapped while the head stayed inside. I tried using gravity, a magnet, shaking and all combined to make the head fall down, to no avail. I've no idea on how to proceed further and am really thinking of getting a new derailleur.

Derailleur with broken head inside hex key socket

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    Tangential - what on your derailleur uses a 0.9mm hex fitting ? The smallest hex on mine is 4mm, plus the limit screws and B screw which are JIS. (ie fancy phillips) Was it trying to adjust a clutch ?
    – Criggie ♦
    Commented Feb 28, 2020 at 6:34
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    @Criggie I’m not sure but I think I’ve seen hex limit screws on some new SRAM or Shimano road derailleur. Which is great in my opinion.
    – Michael
    Commented Feb 28, 2020 at 7:29
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    Ball-end drivers shouldn't really exist below about 2mm, and should be used with real caution and only when needed for access below 3mm. For high torque applications try to avoid them at all sizes. They snap easily and round off quickly, and with shallow sockets in the fasteners (like countersunk, button-head, or in your case a grubscrew) round off the head without leaving much material for a square end to bite. I also have a folding ball-end hex key set; I've chopped off all the smaller balls as they've worn.
    – Chris H
    Commented Feb 28, 2020 at 9:04
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    @shoover it would probably be on topic there, but it's fine here and it's already getting attention so there's no need to change anything
    – Chris H
    Commented Feb 28, 2020 at 17:29
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    I'm just going to call it, that isn't 0.9mm.
    – Lamar Latrell
    Commented Feb 28, 2020 at 19:45

6 Answers 6

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Chances are you were turning it clockwise when it broke. By tapping it anti-clockwise with a small drift or cold chisel, it may unjam it, when a magnet along with gravity and tapping should release it.

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    To add to this, the picture clearly shows that the driver was undersized for the socket, so it has rotated and the corners of the driver are jammed in the faces of the socket. Your solution is the best approach - unjam the driver corners from the socket faces, and it will simply fall out. Since there are gaps between the driver head and the socket, another solution might be to insert a scalpel in a gap to lever the driver head back round.
    – Graham
    Commented Feb 28, 2020 at 10:59
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    @Graham, that picture doesn't clearly show anything. I don't see that at all, and its an over-compressed, over-processed cell phone pic.
    – JPhi1618
    Commented Feb 28, 2020 at 15:02
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    Good point -I'd try a sharp pick, or perhaps some sewing needles to push into the gaps and lever things loose. With some combination of heat and oil it should let go. Its already dead, can't kill it any more, and if OP ruins the mech then it still needed replacing.
    – Criggie ♦
    Commented Feb 28, 2020 at 20:56
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Option 1: You say you've tried a magnet; was it a regular ferromagnet or a rare earth magnet? It might be worth trying to extract it using a strong rare earth magnet. I have a pair of small ones that are about 15mm dia, that are strong enough to cause damage if they snap together with any part of you between them. I'd clamp the magnet in a pair of locking pliers first so it's easier to handle.

Option 2: (Only if option 1 fails) Tool steel is usually pretty hard, but you still might be able to drill it enough with a carbide bit to get a small screw or screw extractor to engage inside it and pull it out.

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  • I suspect that the reason it snapped was that it bit into the steel of the screw and jammed, so I suspect a magnet won't work. But you might be lucky
    – Chris H
    Commented Feb 28, 2020 at 9:16
  • only tried with a regular fridge magnet, didn't have anything better available
    – bookman B.
    Commented Feb 28, 2020 at 9:36
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    Fridge magnets are very weak. You'd have no chance. You can get cheap magnetic pickup tools, which are good, and scrap hard drives (actual spinning disc ones)have very strong magnets in them - mind your fingers - but the shape isn't ideal
    – Chris H
    Commented Feb 28, 2020 at 10:00
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You could try using an angle grinder or Dremel or similar multitool or iron saw to cut a slot into the bolt in which you can insert a flathead screwdriver to remove the old bolt. Then replace it with a new one of the same size. Similar to the method of removing broken bolts (where the head has broken off and the thread are still in the part you want to save for example an engine block) where a slot is cut into the remaining part of the bolt (that is stuck) and this part is removed using a flathead screwdriver..

You could try gluing something (for example the part of the tool that broke off that is still attached to the multitool handle) to the part of the hex key that's stuck and wiggling it out.

Alternatively you could perhaps drill through the bolt ( drill size just a bit smaller than the bolt itself) and chase the threads with a tap of the correct size (when there's just a thin piece of the bolt left after drilling). I would advice center punching the bolt and drilling from the non-bolt-head end of the bolt (for easier centering).

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    From the image I'd say your options 1 and 3 are out - the diameter is too small even of you could actually get at the bolt itself. Rather than chasing the threads you might be able to drill it out and tap bigger, but there's probably a reason it's so small
    – Chris H
    Commented Feb 28, 2020 at 9:09
  • @ChrisH I think option 1 might still work (even though there is a big chance the derailleur's body will be slightly damaged when cutting the slot)
    – Maarten -Monica for president
    Commented Feb 28, 2020 at 9:52
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    I can't even see any bolt to attack with the dremel (and anyway a cuttoff wheel is almost as thick as the bolt's diameter
    – Chris H
    Commented Feb 28, 2020 at 9:54
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    I'm not seeing anything of around 1.8mm, or an accessible bolt head. My Dremel is a cheap fake, and my hand isn't the steadiest, but the cut is always significantly wider than the wheel. That's even true on the precision diamond saw in work. If starting with 1.8mm you'd have nothing like 0.6 either side; you'd be lucky to have half that on the thinner side (and you'd never get it perfect to within 100µm). Last time I tried this I snapped the remains of an M5, where I had over 1mm either side
    – Chris H
    Commented Feb 28, 2020 at 12:08
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    Yes, hammering a torx in is a good option, if you can drill a hole. I've never seen a screw extractor that small, but a left-hand drill bit might do it
    – Chris H
    Commented Feb 28, 2020 at 16:10
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Another option could be to put a tiny drop of CA adhesive (cyanoacrylate, aka "super glue") on the broken end of the hex key still attached to the multi-tool, and then touch that end to the broken end stuck in the socket. You'd want to be very careful with the amount, because if adhesive squirts out of the break then it could attach the broken piece to the socket even more securely. For that reason, I'd try this method as a last resort before replacing the derailleur.

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    Wouldn't try that, it's an excellent way to get the threads completely stuck.
    – Carel
    Commented Feb 28, 2020 at 20:27
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    I would try epoxy instead of superglue, its a lot thicker and less likely to flow somewhere you don't want it, and it generally needs to be clamped to reach full strength: globs that land elsewhere will have rather weak adhesion
    – mbrig
    Commented Feb 28, 2020 at 20:57
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How about a shop-vac? Use some thin tubing, seal everything well with duct tape on the vacuum hose and some oil or something at the other end. With a strong enough vacuum, might work.

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we used a magnet and it worked fine!

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    Hi, welcome to bicycles! Since the question describes that OP tried using a magnet, you need to be more clear about what kind of a magnet you used and how you did it.
    – DavidW
    Commented Mar 9, 2021 at 18:33
  • Welcome to the site - was your situation more like a ball bearing in grease, as opposed to a distorted hex driver end snapped off? Please use edit to expand the answer with some further info. You can also read the tour to learn how SE works.
    – Criggie ♦
    Commented Mar 9, 2021 at 18:47
  • Just had the same thing happen to me, and the suggestion of using a strong magnet out of an old computer hard drive did the trick! Thanks!
    – atbear
    Commented Apr 16 at 16:09

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