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So I attempted using a couple of shitty plastic screws to attach an air pump to the water bottle mounting holes in the frame. I was using a long-arm torque wrench and the head of one of the screws just came off, leaving the body inside.

I need those threaded holes in order to install the pump!

Thinking of burning the screw body with a soldering iron and then forcing a steel screw in, to clean the thread.

Would this be a good idea? Maybe drilling would do better? Anything to be careful about?

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Pick up a screw extractor kit from a hardware store. They're inexpensive and extremely handy. I'd worry that melting the plastic would cause it to stick to the threads of the braze on. –  WTHarper Oct 10 '13 at 12:03
    
@WTHarper, great, I never knew this existed! However, this should not be comment - it is not requesting clarification of the question. –  Vorac Oct 10 '13 at 12:14
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Drill them out. Just use a small bit, so you don't muck up the threads too much. Use successively larger bits until the screw drives on through by itself or it gets thin enough that you can pick it out. –  Daniel R Hicks Oct 10 '13 at 12:36
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Since they're plastic, if you drill a small hole in the middle as @DanielRHicks suggested, and then wedge something in the hole (needle nose pliers?) you could probably get enough grip to unscrew the plastic bolt. Or if you want to melt stuff, heat up the tip of a screwdriver, to make a new head on what's left of the screw. –  Kibbee Oct 10 '13 at 13:04
    
@Vorac bicycles.stackexchange.com/help/privileges/comment I wanted to provide information that I didn't think constituted a complete answer (but I guess technically we shouldn't be discussing community behavior/site policies? I don't know.) –  WTHarper Oct 10 '13 at 22:01
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4 Answers

As the comments have stated, here's your options:

  1. Screw extractor: Inexpensive and easy to use.
  2. Drill: Use successively larger drill bits until most of the screw is gone.
  3. Drill and extract: Drill a spot large enough to put something in the middle and pull it out.
  4. Screw/Nail Extract: Lightly tap a nail or smaller screw into it and then try to unscrew the stuck body.
  5. Melt it: Not really recommended but you could do it. Be prepared to retap the threads if things go awry.

Note, these are not in any order and some can be tried before others.

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You could try heating up a screwdriver and pushing that into the screw, then (after everything has had a chance to cool down) using it to turn the screw out. Start gently and increase the heat until it works.

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I would make the slot and then wait for it to cool down before turning. –  Mark W Oct 11 '13 at 9:03
    
@MarkW I agree - I'll update my answer, as that's what I was thinking of but didn;t write clearly. –  Chris H Oct 11 '13 at 9:41
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The source of the problem was that the plastic body of the screw threads difficultly into the thread. The feeling is comparable to those metal bolts with rubber seal to prevent vibration.

Consequently, when I was screwing the bolt in, there was no increase in the feedback torque - it was moderately difficult to screw the bolt in, up until the head broke off.

So, firstly I tried to burn a slot in the body of the screw with a heated screwdriver. I got it several millimeters deep, but when I attempted to turn, half of the screw broke off.

Next I tried drilling with a dremmel. The bolt is in the bottom of the front triangle, so even though I used a flexible arm (like the ones at the dentist's), I was unable to attack the screw co-axially and the drill bit constantly hit against the aluminum frame.

Next I tried a soldering iron. Bought a cheap, thin-nose, 30W iron and managed to remove a lot of material. This was the most successful method so far ... almost.

Having done everything possible, I inserted a steel screw and tried to chase the thread with it. To my disappointment, it didn't work. The "nipple" at the frame began rotating, with respect to the frame. The bolt become stuck within the "nipple". Now I need to go to a LBS and ask if they could replace the thing.


Bottom line: don't use plastic screws: they are evil. If you must, use a short-arm wrench e.g. 10 centimetres.

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You may have had better luck with a real thread chaser since it would have sharp edges to scrape out the plastic and grooves to let the scraped off plastic out. Since melting likely bonded the plastic with the metal, the metal screw couldn't push it out of the way. –  Johnny Oct 22 '13 at 15:06
    
@Johnny, indeed, I should have listened to WTHarper. Foolish was I, in those days long past: wast week. –  Vorac Oct 22 '13 at 15:09
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Burning plastic could make a big mess. Use precision screws rather than plastic ones. You will have to drill eventually. If you want to go fancy use precision screws made out of titanium parts. They wont break.

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