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I ride a bow shaped handlebar mounted in quill stem. I am riding this with a somewhat sporty setup, which means I do have weight on my hands and thus handlebar grips.

Originally I torqued this to 8Nm. After a first slip, I increased it to 10Nm. This helped, but after riding over a hump and having a peak load on the handlebar, it slipped again.

The stem and handlebar were purchased new, but these are inexpensive unbranded parts, both aluminium and painted rather than anodised. I measured them and the diameters match at 25.4mm. I believe the pinch bolt is M8, and it's steel. After the slip the paint has partially stripped off (see picture). For now I am riding with spare parts, but I want to re-install these if it can be done safely.

How high can I torque the pinch bolt safely?

Additional notes:

  • IIRC I originally used some compound paste on the handlebar/stem interface
  • The pinch bolt was greased
  • It didn't seem to loosen over time, rather only slip on peak load

Quill stem handlebar with stripped paint

EDIT UPDATE: I removed the paint from the center of the handlebar with sandpaper, which also did not help sufficiently. I again torqued up to 10Nm, which was not enough to prevent slippage. I dared not to go higher as the handlebar is aluminium. I decided against a shim, and instead tried using another stem I had on hand. It is 10mm longer, but I installed it slightly higher and that was enough for it to be comfortable. The handlebar has not slipped in the other stem in the last weeks, so I hope this was sufficient for now. Will replace the handlebar when I find a suitable replacement just to be safe.

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  • Would have thought 10nM be plenty. Maybe some assembly compound might help?
    – Hursey
    Jun 2 at 2:32
  • If I recall correctly, I originally applied some when it was torqued down to 8nM. By habit I never install parts without anything. It's dry now, cause I wiped it to review the stripped paint which is why I am not 100% certain what I used. Jun 2 at 2:38
  • How much thread engagement do you have on that pinch bolt? M8 should be able to handle more than 10Nm if there's enough threads. Also, does the paint wear pattern on either mating surface look uniform? If the paint's only gone in one area, that may indicate a high spot and vice versa. For diagnostic purposes you could also try installing the bars again and seeing how much force it takes to make them slip. If it's crazy difficult after a fresh install, perhaps the clamp bolt is indeed loosening.
    – MaplePanda
    Jun 2 at 4:58
  • Carbon grip paste is very helpful in these scenarios.
    – Noise
    Nov 23 at 15:08
  • Thanks @Noise, that's actually what I used when I referred to "compound paste". Nov 24 at 2:49

2 Answers 2

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The paint will be contributing to slipperiness in the interface initially, but looking at the wear seems there has also been fretting which has eroded metal.

I suspect the inside of the quill stem clamp area is also equally smooth, either by design or by wear.

Some options to try:

  • More clamping pressure on the nut. This may strip the thread, snap the bolt, stretch the bolt. It will also contribute to the "lapping-compound" effect that is wearing aluminium off the mating surfaces. Not recommended.
  • Clean the interfaces - scratch out all the paint in those grooves. Make them more-defined. Do the same inside the bore of the quill stem.
    Your goal here is to make a more "toothed" interface to discourage movement.
  • Add more friction by using a small piece of double-sided sandpaper as a shim. This should bite into the aluminium on both sides and resist movememnt. You might be tempted to add grease and sand, but this allows too much movement of individual particles.
  • Add a shim of frangible material - Aluminium Can has worked well for me on some drop bars with the same issue. The shim can deform easier than the bars or stem, and will lock in a bit better.
  • Replace the quill stem with something more modern? Forgo the vintage look and put in a threadless steerer with four bolts holding the bars in place via a cap, compared to existing compression clamp with one bolt.

Example: enter image description here

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    Thanks for the suggestions! I'll address some in the comments. "More pressure", this is indeed what I'd like to try but not sure how far I can go to stay within reason. Hence the question (I put in bold now to clarify). If 12nM is considered safe, I suspect it will do the trick. Jun 2 at 4:22
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    "Clean the interfaces" I will try this first. The inside of the stem is indeed also painted. I'll try to sandpaper this, at minimum to rough the top coat. Perhaps remove the paint in the grooves with a utility knife. Jun 2 at 4:24
  • "Add shim or sandpaper" I was hesitant about this, as the bar and stem did measure to the correct size and I am not sure what the tolerances to keep it safe (0.1mm??). Perhaps after removing some of the paint it's even needed to have a shim. Jun 2 at 4:26
  • @Superman.Lopez "shim" was a descriptive word - you're not trying to fill space, you're trying to add friction. There are other more-brutal solutions but I'd think twice before applying them to my own bike, let along recommending to someone remotely, like the nail insert, or subtly-deforming the clamped section. I'd absolutely NOT drill and bolt anything no matter how tempting.
    – Criggie
    Jun 2 at 11:56
  • Thanks @Criggie. In that case: would you recommend inserting a soda can shim of which the length is as wide as the stem and maybe 1cm in width? Or what size did you have in mind? Jun 4 at 1:52
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Not an answer, but I cannot post pictures in the comment...

Please be extra careful with incrementing torque. M6 bolt will take a lot, but single-bolt interfaces are more suitable for steel handlebars. You can crush aluminium handlebar by applying too much torque. The biggest issue is that damage is unnoticeable untlil you ride a bike. Handlebar failures due to overtightened stem are sudden and catastrophic.

My own experience with an overtightened modern stem:

enter image description here

From the interwebz (stem more similar to yours):

enter image description here

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    Thanks. I am certainly concerned about this and will be careful. Jun 4 at 1:53

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