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I was fastening Shimano STI levers onto Bontrager carbon drop handlebars yesterday, with my torque wrench set at ~2nm, and heard a short rather loud click/snap sound shortly after starting to tighten, probably well below 2nm at that point. I took the levers off, and inspected the area by shining a flash light on it and looking through a magnifying loupe, and also tapped the area with an allen key to see if there was any change in sound. It looks and sounds fine.

If not necessary I'd prefer not to throw them away, but I also don't want to take any big risks. Does it sound like my bars are a write-off?

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    Have you a) applied carbon paste between collar and bar and b) greased the tightening bolt. A dry bolt falsifies the torque readings. And it is very difficult to tell if a carbon part is or is not compromised. By X-ray maybe. In case of doubt I'd refrain from further use, especially with a handlebar.
    – Carel
    Oct 2, 2017 at 7:31
  • A: Yes, I applied Park Tool SAC-2. B: I did not grease the bolt.
    – Fjölnir
    Oct 2, 2017 at 7:55
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    Compromised carbon parts are likely to snap without warning. Potentially lethal in this case!
    – Carel
    Oct 2, 2017 at 12:18
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    Yeah you're right. I went and got nice aluminum bars and put the carbon ones in the closet in case until I get an opportunity to have them tested properly.
    – Fjölnir
    Oct 2, 2017 at 13:24

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