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I have a Allen bolt which has a domed head. Unfortunately the head is rounded. The location of the bolt means I can't get in to make a cut and remove it with a screw driver.
It is mounted to some plastics so I don't want to hammer into it in fear of cracking the plastic.

I'm thinking of filling the hole in the bolt with some metal putty (like Loctite ultra repair epoxy) and making a screwdriver in that epoxy. Once hard I should in theory be able to unscrew the bolt.
Is this a viable idea? Would it work?
Cheers

Edit took a picture today of said bolt. I have tried drilling it out with left hand bit, here's what I'm dealing with:

enter image description here

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    I am skeptical that the putty would work. Depending on the location it may be possible to drill a hole and use an ez out (aka easy out) screw/bolt extractor. If the bolt is stuck in place due to rust, then it may also be helpful to treat the area with penetrating oil (Kroil etc). Commented Jan 31, 2018 at 19:43
  • I do have a left handed drill bit to extract but the bolt is an m6 and the bit is a 3mm so that may be too small. The bolt isn't rusted and is just over hand tight so it doesn't need that much force to get it out.
    – DamoC
    Commented Jan 31, 2018 at 20:12
  • Post a picture of the situation please, would go a long way to visualizing the bulk of the relevant constraints.
    – jxramos
    Commented Feb 2, 2018 at 1:32
  • are you sure this isn't a left hand thread?
    – Martin
    Commented Feb 4, 2018 at 20:23
  • 100% certain the other 3 bolts have come out fine.
    – DamoC
    Commented Feb 4, 2018 at 22:24

5 Answers 5

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Get a set of easy outs. If you think you can get to it with a drill this may be the best option.

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Is the inside part where the Allen wrench goes is completely rounded out? If you can get any purchase in there, find a flat tipped screwdriver which fits snugly in the whole. I've found you can usually get it to come out that way. Just force it gently into the hole. If you have a screwdriver which is too wide, but not afraid to do a little grinding on, you can shave the sides down (either with a grinder or file) until it fits. Then turn the screw out. Even if the hole looks like there won't by any purchase, there's usually more there than you'd think.

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  • It is pretty much complete. There is some play to get screwdriver in there but the amount of hamering to get it in I fear I may break the plastics. Grinders are out of qustion as the bolt is recessed.
    – DamoC
    Commented Feb 1, 2018 at 17:06
  • @DamoC - You really didn't understand what I was talking about. Don't hammer the screwdriver in, just press it in there. As for grinding, grind the screwdriver to fit the hole in the bolt head, not the bolt head itself. You really should try this method. Saved my bacon more than once. Commented Feb 1, 2018 at 17:59
  • I see, sorry for the misunderstanding. I'll give that a try Cheers
    – DamoC
    Commented Feb 1, 2018 at 18:16
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In addition to easy outs and hammering a screwdriver in, which are both good suggestions one possibility would be to use a rotary tool and cutting disc to cut a groove on the head that you can get a large flat head screwdriver in. That is only going to work if you can get the tool in laterally, although you can get flexible extenders that let you get the discs into pretty inaccessible locations.

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  • I had thought of using a dremal but The bolt is recessed and the dremal would cut into the plastic.
    – DamoC
    Commented Feb 1, 2018 at 17:04
  • No cutting disc options there then, I suppose you could use other bits to modify the screw head. It's worth keeping it in mind anyway.
    – GdD
    Commented Feb 1, 2018 at 18:49
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There ought to be left hand drill bits available. With those the chances for an successful extraction should be higher

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  • I have a left handed bit at 3mm, I believe the thread on the bolt is an m6. Would a 3mm bit be big enough?
    – DamoC
    Commented Feb 3, 2018 at 8:49
  • @DamoC I suppose it ist worth a try. With a bit of luck the bolt goes out, if not you have a nice guide for a larger diameter. Just make sure you drill it exactly in the centre
    – Martin
    Commented Feb 3, 2018 at 11:34
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Most of the screw extractors I've seen have been of the left-hand-thread variety, which I've found less successful than just a left-hand drill.

But a trick I learnt from the workshop here has proved very successful:

  • Drill a hole into the screw, on axis (a little over half the nominal screw size seems reasonable)
  • Then take a bolt and make it into a square taper using a bench grinder or similar.
  • Press this into the damaged screw (or gently tap with a hammer), then unscrew.

With a damaged socket head screw you can often make a square taper that you can hammer into the head, but that would probably take more force than you're prepared to apply. Gluing in ito the head with epoxy might work, but I'm always wary of doing this in awkward spaces in case the glue gets somewhere it shoudln't and makes things worse.

A hex-head screw works well, with a socket to drive it, as it makes holding everything straight easier.

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