Sheared Stem Bolt

dee4life2005
dee4life2005 Posts: 773
edited July 2013 in Workshop
My headset had developed a little bit of play after riding over a cobbled section of road a few days ago.

I loosened the stem bolts, adjusted the headset bolt, and then tightened the stem bolts back again ... to 4.5-5Nm ... the stem says max 6Nm, but it's a carbon steerer so I reduced this a little. I tightened the stem bolts using a recently calibrated torque wrench, doing each a little at a time alternating between them. At a little over 4Nm one of the stem bolts sheared clean in half, which resulted with the head part coming out of the stem, and the end of the bolt stuck in the threaded part.

Thankfully there was just enough of the end of the bolt sticking out to hacksaw a slit in the end and use a screwdriver to wind it out.

This is a standard stem bolt, not a titanium one ... was this just bad luck, or is there any of the above that I've done wrong that may have caused this.
http://app.strava.com/athletes/307297

Comments

  • estampida
    estampida Posts: 1,008
    well it depends on whether its a machine tread or rolled tread

    machine 1's can fail quite often, but a rolled thread failing...... not that common

    what does the shear look like (is it a spiral shape and failed in the middle, jagged line and failed on the outside.... and so on)

    after that its what caused it? and any remedial work to stop it happening again.
  • dee4life2005
    dee4life2005 Posts: 773
    It's a fairly clean break and it looks to have failed in the middle in a kind of spiral shape as it's failed along one revolution of the threads.
    http://app.strava.com/athletes/307297
  • Monty Dog
    Monty Dog Posts: 20,614
    It happens - that's why most stems use four. May be worth replacing all four as a safety measure
    Make mine an Italian, with Campagnolo on the side..
  • dee4life2005
    dee4life2005 Posts: 773
    I could understand if this was one of the four bolts that hold the stem to the handlebars as they must get some force when pedalling out of the saddle. However, this was one of the two bolts holding the stem onto the steerer tube and keeping the pre-load on the bearings in the headset.
    http://app.strava.com/athletes/307297
  • crankycrank
    crankycrank Posts: 1,830
    Could just be a defective bolt or it was possibly overtorqued at another time and weakened. Check the hole where the bolt head is supposed to rest flush against in the stem. If it has a burr or some type of deformity this could fatigue the bolt as well. If all looks good just replace the bolt, check it once in awhile and don't worry too much about it. Defective bolts are rare but do happen.

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