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In the latest episode of my disastrous bike maintenance attempts, I got a sub-standard torx bit that broke off in the bleed screw of my Tektro Vela disc brake caliper.

Here's a sligthly overexposed picture of the brake caliper in question, the bleed screw is circled in red:

Image of the screw in question

And another shot from the side. Notice how the bleed port is recessed rather than the bleed nipple you'd usually see on a Tektro brake.

Side-view of the brake

Considering that the screw is slightly recessed into a somewhat expensive safety device, I'd like to find out a way to get the bit out in the least destructive way. If all else fails, I'm considering eating crow and taking my bike to the LBS in the hope that they'll have the tools, experience and/or spare parts to fix this.

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    Could you post a photo from different angle, sideways? – krzyski Dec 21 '17 at 12:20
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    @krzyski I added a second picture, that's not the typical tektro bleed port (it's shaped like a torx screw head, for one) – HAEM Dec 21 '17 at 12:46
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    @krzyski I'm fairly certain that there's just enough of the bit stuck in the slot to keep a second bit from finding purchase. I guess I can try after I get a replacement bit. – HAEM Dec 21 '17 at 13:47
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    Somehow position things so the screw is facing straight up. Clean the bolt head with soap and water and a toothbrush. Rinse well, dry well, and then clean again with rubbing alcohol. Hit it with a hair dryer for a couple of minutes to drive out all the water you can, then drip in some oxalic acid solution or other "rust dissolver". Let soak for 30 minutes, dry out, hit it with the hair dryer again, and then try to pick the bit head out with a needle or the tip of an X-Acto knife. – Daniel R Hicks Dec 21 '17 at 19:37
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    What about a magnet? You might try tapping it gently with a small pointy object (I have an old nail-set that I would use, maybe a small nail would work) to try to loosen it a little bit. If you have a nice strong magnet it might be able to pull it out. – Drew Smith Dec 21 '17 at 20:49
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If the bit broke from being defective and not from too much torque, it may be as simple as turning the bike upside down and gently tapping the caliper with a hammer.

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  • Sadly, some of the dents you see are from me trying to knock the bit loose by knocking on the caliper with a wrench. Of course, if this is a case of too much torque, I have to wonder how they managed to tighten the screw in the first place. – HAEM Dec 21 '17 at 20:59
  • I ended up having to detach the brake caliper and knock it against the head of a rubber mallet a couple of times to get the bolt out. – HAEM Dec 30 '17 at 13:21
3

From the photo you posted reasonable option is to drill a hole in the bit and use drill extractor, spinning it anti-clockwise.

Bit is made of hardened CrV steel, so drilling it may be hard, since it should be precise and centered. If you feel unsure about your skills, give it to professionals, to locksmith or to your local bike service. It may appear cheaper, since damaging caliper housing would be much more expensive than spending some extra euros to have it fixed.

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2

Do you have access to a drill press?

Good answer from krzyski but you don't need to extract.

Start centered and use bigger bits until the torx bit fractures.

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  • I didn't downvote but it might be difficult to get the brake caliper into a drill press and clamp it securely enough to drill out that screw -- without damaging the caliper. – RoboKaren Dec 26 '17 at 21:26
  • @RoboKaren Really? How did you resist another opportunity to down-vote me? I was thinking an eccentric drill press. You think the accepted answer has some magic here? – paparazzo Dec 26 '17 at 21:39
  • I only downvote questions and answers that don't contribute. If you didn't post a drill press response, someone else would have. I thought of it as well, but I don't see how you'd get the part into a drill press vise without damaging it. So my comment was left as a note for people who might think of trying it - I think it'll be trickier than it looks. – RoboKaren Dec 26 '17 at 21:41
  • I'm also not convinced the accepted answer is the right one either. It's a very small bolt and drilling out the head (especially with a hardened bit in it) is going to be tough. So I didn't upvote that answer either. – RoboKaren Dec 26 '17 at 21:42
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I would try alternating heat and cold.

Heat from a hair dryer (since your parts are painted, you can’t use a propane torch) or the tip of a soldering iron.

Cold using an upside down “canned air.”

It’s even better if you can get some moisture underneath the broken torx bit as you heat it up with the soldering iron tip. Wear eye protection as the bit can fly loose.

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  • But note that any moisture under the bit will create adhesion. – Daniel R Hicks Dec 25 '17 at 15:20
  • Once the moisture reaches boiling point it’ll create pressure behind the bit. That and differential expansion will work it loose, sometimes violently. – RoboKaren Dec 25 '17 at 16:07
  • You think this a perfect seal to build pressure? How? This is a jam on two metal parts. – paparazzo Dec 26 '17 at 21:43
  • I've done this on broken bits (allens and torx) a few times. Not clear if it's the differential expansion or the liquid evaporating behind the bit, or something else, but it does work. – RoboKaren Dec 26 '17 at 22:53
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    In the end, I didn't need to go this far. It was enough to bring the caliper to room temperature and knock it against the head of a rubber mallet a couple of times to get the screw loose. – HAEM Dec 29 '17 at 15:45
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It's a tiny bolt (maybe only around 5-6mm across) so drilling it out and getting a screw extractor in might be a bit tough.

You might consider using a dremel rotary tool with a thin drill or abrasive diamond bit. You want to drill a small hole in the center to get the screw extractor in -- or perhaps more realistically a straight trough into which you can fit a flathead screwdriver so that you can unscrew the bolt.

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If there's a way to get out the broken bit with a suitably powerful magnet, that's got to be by far the approach that causes minimal further complication. Getting a replacement bleed screw for this brake may be pretty difficult (or Tektro may just have one to send, who knows). It is critical to use the right screw and o-ring so that the seal works.

Cow magnets are pretty strong and readily available, and the right shape. Of course a pickup magnet or computer magnet may be more readily on-hand. There may be some creative way you can figure out access to an electromagnet.

Drilling into the tool steel of the broken bit without destroying everything else does not seem very realistic to me.

In theory you could find something to bond a tool to the screw and broken bit, unscrew it, then use heat to undo the bond. In practice whenever I try something like this the bond doesn't really get strong enough.

Using an adhesive to pull on the broken bit is pretty likely to get it stuck in worse, although it's true that in theory this might be avoidable. You have the advantage that the broken parts probably want to mate closely and provide lots of surface area. You might try cleaning the surfaces with alcohol on a q-tip, put a very tiny dot of epoxy on the connection point, sticking it on and securing with a rubber band, and leaving it for its full cure time. If you can avoid using enough epoxy that any spills out the crack of the bit, you should be safe from making it worse this way.

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  • You wouldn't be heating the brake, just the bleed screw once it's extracted. – Nathan Knutson Dec 22 '17 at 0:30
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Depending on how stuck it is you may be able to pull it out with super-glue on the end of a stick. Use a "helping hands" tool to hold it while it sets.

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    With something that small, superglue sounds like a good way to superglue the bit in even more permanently if the superglue runs off the edge of the bit. – Johnny Dec 21 '17 at 18:40
  • I'd suggest blu-tak or even chewing gum over superglue. – Criggie Dec 21 '17 at 19:14
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    Based on my experience with superglue and metals, this isn't going to work. – Mark Dec 22 '17 at 0:08
  • Superglue won't hold well. That's why it depends how stuck the bit is. The upside in this case is the break surface is new and clean. The surrounding parts are dirty and oily. The glue really isn't going to stick well there. So don't clean it first. A downside is the break surface is probably smooth. – dreamcatcher Dec 24 '17 at 11:09

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