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I've been working on getting my mirrors to sit still when riding. They screw into a rubber block that presses out against the inside of the handlebars which backs off over time.

So I bought some small star nuts and they seemed perfect for this purpose.

https://www.tartybikes.co.uk/headset_spares/tartybikes_star_nut/c57p10541.html
from https://www.tartybikes.co.uk/headset_spares/tartybikes_star_nut/c57p10541.html

Installation went fine, using a suitable sacrificial bolt and some firm taps with the hammer. BUT on tightening the bolt, I felt the tension drop, and felt and heard grinding noises from inside.

Turns out the whole star nut was spinning like a blade inside the bars, cutting a slot and producing aluminium flakes.

What are my options for saving this? The bolt won't back out, and there's not enough space to get something in and hold the starnut.

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  • In my case this is installed in the end of handlebars, but its functionally the same as a star but inside a threadless steerer, albeit smaller.
    – Criggie
    Jul 12 '20 at 1:33
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    I suspect they're too small. Jul 12 '20 at 2:43
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    Can’t you drill them out? The only other option I can think of is to thread a wire (or sturdy string, shifter cable etc.) around the “star” spikes and pull on it to disengage them. I assume you’ve installed them on both ends of a handlebar and can’t just simply push it through to the other side?
    – Michael
    Jul 12 '20 at 5:15
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    Aha! So you can’t even uninstall the mirror anymore which is also the reason you can’t insert anything to hold the starnut. In that case the only option I can think of is to gently clamp the handlebar with a vice where the starnut is installed to cause some elastic (non-permanent) deformation to hold the star nut in place. Dangerous for the handlebar though.
    – Michael
    Jul 12 '20 at 8:18
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    One other thing I just thought about: Is the other side of the handlebar open? Can’t you drop a tiny nut (as a weight) tied to a string into it, turn the starnut, hope it tangles itself up around the star’s spikes, then pull on the string to hopefully hold the star nut in place?
    – Michael
    Jul 12 '20 at 8:22
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To resolve this situation, there are several options each with advantages and disadvantages (ie risks)

In my personal case it was a combination of multiple causes

  1. The star nut hadn't been quite hammered home far enough, so the second line of "leaves" was still on the outside of the tube,
  2. The tube was an aluminium handlebar that had been trimmed short. So the internal wall thickness increases toward the center, unlike a bent steel handlebar.
  3. The ID had been reamed out in the past to try and give more space for clamping the old rubber handlebar mount, but only to a depth of about 20mm. The starnut had got to that lip and not gone any further.
  4. Due to a turn in the bars, the mounting bolt was a little too long, and ended up threading through the starnut and bottoming out.
    This pushed the bolt end sideways before it was tight, forcing the starnut back out a little, giving it less purchase on the walls and then it was free to spin and grind the inner walls of the bars without backing out. I had locktite on the threads too, which didn't help the bolt to backout, even though it had not set.

Answer:

  1. If you can't get the bolt to undo and you can hear the starnut turning, attempt to lock up the starnut by pulling it sideways, and twisting. Assumes the topcap is not squashed down onto the steerer.
  2. Another suggestion was to gently "crush" the outside of the tube so it constricts the starnut. However this would be immediately fatal for a carbon part, really bad for an aluminium part, and only has a chance of succeeding with steel which will probably be permanently deformed by this action. I didn't even try this.
  3. If there's some level of bolt outside the top cap, you might get lucky by hammering it into the steerer, to help seat it enough to resist the bolt's undoing torque.
  4. Decide what's more important or harder to replace, and use a drill to remove the other thing. Drilling through the bolt cap would let you save your top-cap, or the mirror's arm in my case.
  5. Once you can see the star nut then there are many more options.
  • Simply seating it further into the steerer tube will probably secure it nicely.
  • Do inspect the ID for scoring though - if its cut badly then this steerer and probably the whole fork is toast and needs replacing.
  • If its a straight steerer, you may choose to drift the starnut down and out the bottom of the fork. Though this will likely ruin the starnut.
  • Lastly, you can use a drill bit and drill driver to shred the core of the starnut. Once sufficiently thinned, the whole thing should fold up with long pliers and drop out by gravity.

Here's the bolt that had to be thinned and rethreaded to fit the starnut. It was originally an M8 bolt for a motorbike. enter image description here

Here's the starnut after it had been hammered home properly - there's a good 5mm of overlap now. I also gobbed in a lot of epoxy to help lock things down, and then cleaned up the threads with an M6x1.0 tap. enter image description here

Other options that I didn't try:

  • drilling through the bars to offer access to the starnut. I felt this would weaken the bars even more right at the point where it was scored.
  • Trying to feed something in from the other end - with a steerer tube this might have worked, but mine was a wiggly set of bars with no real access. And the other side had a star nut in it already.

One Month Later

The starnut failed internally after I whacked the mirror on a railroad crossing while negotiating a chicane. The threaded core came loose from the two spring-steel "flowers"

I tried peening the flange to resist movement, but spring steel isn't malleable. I considered epoxy but that was going to be hack.

Since I had a spare, the only fix was to remove the whole thing and replace. I drilled out the core's flange, using a 9mm twist drill at an angle so the core wouldn't rotate. Once they were sepatate, a 10mm twist drill caught the first spring steel flower and pulled it right out. An M6 bolt in the core and a crowbar was enough to yank the the core out. The inner flower got pushed well into the handlebar and is now unreachable around a curve, so there it stays.

The replacement starnut got scratched up with a file, and then I slathered the entire thing with 2 part epoxy before hammering home like the previous one. The epoxy should help resist the rotational forces.

Future? If it fails again, I will turn a ~20mm plug of aluminium, drill and tap for a M6 bolt, tap it into the bars and pin in place through the bars, under the bartape. I might even contemplate using a left-handed thread for the main bolt.

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