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Talk me through removing a rounded crank arm bolt

eddiefiolaeddiefiola Posts: 344
edited March 2015 in Workshop
Rounded the non drive side off, completely! Tried a torx key, tried the next size up allen key, tried hammering a screwdriver in, it's long gone.

I assume my only bet now is a screw extractor but never used one before. Have ordered a set from Draper and a T type tap wrench.

Looked on youtube and not sure if I need to drill in the middle of the rounded bolt first (what size drill bit?) or whether I can just go straight in with the screw extractor, any do's and don't appreciated.


  • PokerfacePokerface Posts: 8,640
    What type of bolt is it? Different cranks have different bolts, etc. Or what cranks are they?
  • Shimano Ultegra 6700. Hex bolt 5mm
  • JayKostaJayKosta Posts: 635
    The screw extractor set should (might?) have instructions about drill size and how-to-use.

    There needs to be a hole in the bolt that is deep enough for the extractor to securely grab onto. If there already is a usable hole in the bolt, then fine - otherwise you'll need to drill.

    I suggest you put a few drops of 'penetrating oil' on the bolt to help lubricate the threads and reduce any corrosion.

    Jay Kosta
    Endwell NY USA
  • crankycrankcrankycrank Posts: 1,830
    If you haven't tried this already make sure the bolt next to it is torqued down properly or even slightly tighter to take some of the pressure off the stripped bolt before trying to unscrew it. If no luck with this try drilling the bolt head off first and make sure you drill a little past the head in case you need a screw extractor later you'll have a nice indented starter hole for a smaller drill bit. With the bolt head gone the bolt should unscrew easily by turning with a pick or tool of your choice to turn the bolt. If no luck use an extractor bit.
  • FudgeyFudgey Posts: 892
    It might be a bit of a censored to get out cleanly in all honesty

    I quite often have to deal with sheared screws in alloy at work and so far have not been successful in using an easyout etc as i always manage to snap,them in a small size such as M5.

    If you are very careful you might be able to drill it out without going into the alloy of the crank arm, or if it goes slightly off centre you can re tap it after. Or drill it out, then drill to 5.5mm and tap to 6mm. Might then need to drill out the crank arm slightly larger where the head of screw will go.

    For penetrating fluid use plus gas, NOT WD40 - the WD stands for water displacer! It is not a penetrating oil.

    Why are you taking it off by the way? If it is to replace it then just drill the head off the screw.
    My winter bike is exactly the same as my summer bike,,, but dirty...
  • Where are you based? More than willing to give you a hand of you are local to Portsmouth. Otherwise I would take it to a local bike shop or engineering work shop
  • eddiefiolaeddiefiola Posts: 344
    So I finally got around to doing this as my BB needs replacing.

    Drilled about a 5mm wide, 5mm deep hole in the center of the crank arm bolt, then used a Draper screw extractor and hand tool to turn the bolt in the opposite direction, luckily it came free no problem.

  • keef66keef66 Posts: 13,286
    Thanks for taking the time to update us. Always like a happy ending.

    Is that a massive bolt or a tiny Dachshund?
  • eddiefiolaeddiefiola Posts: 344
    haha just perspective, he's not that tiny, just the bolt is so close to the phone camera lens.
  • veronese68veronese68 Posts: 22,304 Lives Here
    Well done, that was lucky. Have to agree with what Fudgey said the small extractors like that are very easy to break. I know several people that have broken them.
  • FudgeyFudgey Posts: 892
    Good effort!
    My winter bike is exactly the same as my summer bike,,, but dirty...
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