Left hand crank fell off twice on way home....

barry_badrinath
barry_badrinath Posts: 154
edited March 2017 in Workshop
Hello All. As above, my left side crank came loose a couple of times on the way home last night. This was after I had swapped it onto my other bike for a ride on Saturday and then put it back, so it's likely 99.9% down to my mechanical ineptitude.

I managed to get home last night after tightening it up sufficiently, but now it looks like the small plastic thing in the middle that pops out has broken, and also looks like there should be some kind of o-ring or washer there that must have been lost when it came off.

Basically, should I get someone to look at this, and will it keep coming loose if I don't? It's a Shimano 105 hollowtech crank.

Comments

  • Bobbinogs
    Bobbinogs Posts: 4,841
    Barry Badrinath wrote:
    ...
    Basically, should I get someone to look at this...

    Yes!
  • keef66
    keef66 Posts: 13,123
    How were you tightening it? A Hollowtech crank shouldn't just drop off. You can buy a replacement preload cap for a couple of quid.

    What you should be doing:

    Put the crank arm on the end of the spindle and push it home till it's snug, then tighten the little plastic cap only finger tight to apply a small preload to the bearings. Then you install the 2 pinch bolts with their washers, and the little plastic safety tab which goes into the gap in the crank and it's pin sits in a hole in the crank. Tighten the pinch bolts alternately / evenly. No O-rings involved.

    Since yours has dropped off twice, it's possible you've damaged the splines on the inside of the crank. Have a good look at it before attempting reassembly again.
  • cooldad
    cooldad Posts: 32,599
    There is an o ring that fits around the spindle and into a recess in the crank.
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  • jermas
    jermas Posts: 484
    The black pastic part that's broken is called a stopper plate. It's there to help align the crank arm when installing. You can install the crank arm without it. As above tighten the bolts evenly a small amount each time. They need to be pretty tight (12Nm- 14Nm)
  • keef66
    keef66 Posts: 13,123
    cooldad wrote:
    There is an o ring that fits around the spindle and into a recess in the crank.

    On the non-drive side? Not on my 105 there isn't. (Off to look at Shimano tech docs to make sure I'm not missing something...)
  • keef66
    keef66 Posts: 13,123
    Oh, yes there is! One each side. Maybe it's stuck in the crank arm...
  • Alex99
    Alex99 Posts: 1,407
    Barry Badrinath wrote:
    Hello All. As above, my left side crank came loose a couple of times on the way home last night. This was after I had swapped it onto my other bike for a ride on Saturday and then put it back, so it's likely 99.9% down to my mechanical ineptitude.

    I managed to get home last night after tightening it up sufficiently, but now it looks like the small plastic thing in the middle that pops out has broken, and also looks like there should be some kind of o-ring or washer there that must have been lost when it came off.

    Basically, should I get someone to look at this, and will it keep coming loose if I don't? It's a Shimano 105 hollowtech crank.

    What are you tightening? Not the correct thing it seems.
    " tightening it up sufficiently"
  • barry_badrinath
    barry_badrinath Posts: 154
    Well you can probably see that I am not much of a bike mechanic! I was tightening the two pinch bolts as suggested above. It does seem like the o-ring is gone and the 'stopper plate' funked. I reckon I'll get someone more qualified to take a look...
  • keef66
    keef66 Posts: 13,123
    I'm pretty sure the o-ring on my 105 is sitting in a recess in the LH crank arm. Certainly don't remember fishing one out; I've swapped 3 chainsets between 2 bikes about half a dozen times and never had to retrieve an o-ring.
  • Alex99
    Alex99 Posts: 1,407
    Barry Badrinath wrote:
    Well you can probably see that I am not much of a bike mechanic! I was tightening the two pinch bolts as suggested above. It does seem like the o-ring is gone and the 'stopper plate' funked. I reckon I'll get someone more qualified to take a look...

    The two pinch bolts are what hold the crank on.

    There should be a small rubber/metal ring that sits between the crank arm and the bottom bracket. If it's a triple, there will be a larger black spacer / washer that goes between the left crank arm and bottom bracket. Still even without either of these, the crank shouldn't fall off. :?

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