Stripped Crank Arm

Discussion in 'Cycling Equipment' started by rcrampton, Jan 1, 2008.

  1. rcrampton

    rcrampton New Member

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    My left pedal stripped right out of my crank arm. Strange because I'm sure I've torqued my pedals about right (20 ft-lbs) and I've never had them come loose. Incidentally I changed pedals just before the ride so I know it went in smoothly and was torqued.

    Anyway, my LBS wants to chase the threads with a tap. I have a mental picture of being on a tour in the middle of nowhere and having my pedal strip out again. I don't believe chasing the threads makes sense, there were metal shavings in the pedal threads, I'm concerned there isn't enough thread left to be reliable.

    My question is... Can I put a helicoil in a Shimano Deore 26/36/48 crankset (purchased Feb of last year)? Are the crank arms solid aluminum or hollow? Is there enough metal there to hold a helicoil? I *know* those won't fail!
     
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  2. gclark8

    gclark8 New Member

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    Helicoils in LH thread are out there (BiCorp in Aus), however a new LH crank arm is probably cheaper after considering LBS labour. ;)
     
  3. kdelong

    kdelong Well-Known Member

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    Of course if you have access to a drill press, you could probably do it yourself for a fraction of the cost of a new crank arm. You could probably do a decent job without the drill press, but use one if you can to ensure that the hole is square to the axis of the crank arm.
     
  4. rcrampton

    rcrampton New Member

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    I have a drill press and should be able to rig up a nice hole. I know Helicoil makes left hand threads (hopefully the 9/16-20 for pedals), so I should be OK there.

    My real worry is... is the crank arm solid or hollow? If it's solid I'm good to go, the Helicoils will be stronger than the original aluminum threads. If the crank is hollow I'd like to know that there's enough material around the pedal thread area to hold the helicoil reliably.
     
  5. leestevens

    leestevens New Member

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    Mate, for the amount of material you will take off for a heli-coil there will be plenty of metal left, even if the cranks are hollow:).
     
  6. CAMPYBOB

    CAMPYBOB Well-Known Member

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    If it's solid I'm good to go, the Helicoils will be stronger than the original aluminum threads.

    Yes, the threads will be stronger due to the Helicoil or Keensert being made of steel, but the crank arm eye will be weaker due to material removal.

    http://pardo.net/bike/pic/fail-001/FAIL-001.html

    Cranks arms are cheap when compared to the possible consequences of breaking one.

    Just sayin'! YMMV, of course.
     
  7. kdelong

    kdelong Well-Known Member

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    Every crank arm that I have broken was solid. It has been quite awhile since I broke one though, but I haven't heard of any being hollow. Shimano Hollowtech might be but I don't think the crank arms are.

    Although the stainless steel helicoil threads will be stronger, they are still being held in by alloy threads so it doesn't really make the pedal mounting any stronger. I am not an expert in these matters but I have to disagree with Campy Bob on this one. I cannot see where the crank arm will be weakened to the point that it would be dangerous. Very little metal is being removed. If anyone disagrees with this and can back it up with facts rather than anecdotal evidence, please post as I am always willing to learn.
     
  8. rcrampton

    rcrampton New Member

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    Thanks for the comments guys. I can say that a helicoil will improve the strength of the thread system overall (their web site has some explanations I believe). We use this on race cars with aluminum intakes and heads. If you are putting bolts into and out of these a lot you'll see thread failures. So we just drill them out and helicoil them right out of the box. Once the helicoils are in it's much more rare to see a failure.

    For some reason I recall being told by someone that Shimano made some hollow cranks. I think mine are too heavy to be hollow, I suspect they were mistaking "hollowtech" with meaning hollow construction, which I doubt they are.

    I'd be comfortable with the reliability of the crank with a helicoil, but as always I'll get some miles on before the first tour!

    Thanks y'all.
     
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