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My bike's adjustable stem keeps falling down. I tighten the nut as much as can but it seems no impact at all.

Is it the wrong nut to tighten?

enter image description here

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    The part is called stem. Headset is the bearing that allows the fork and handlebars to turn. These adjustable stems are in general a common source of problems and best replaced with non-adjustable ones. You'll need a headset top cap to install a new stem. – ojs Oct 19 '19 at 13:42
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    Also - check underneath for a second clamp - you may simply be tightening the pivot/axle and the real clamping point is a plate underneath. Or it may have fallen off in the past. – Criggie♦ Oct 20 '19 at 3:04
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    Interesting angle on the brake levers 🤔. Do you have the stem angled near vertically usually? – Lamar Latrell Oct 20 '19 at 8:57
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There are quite a few adjustable stems where you would NOT tighten the correct bolt.

Also tightening this bold too hard can lead to a catastrophic failure of the stem.

For quite a few of those adjusting stems the tightening bolt is underneath.

stem with tightening bold below

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    Yep, I'm fairly sure I've seen several different styles of adjustable stems where there is some sort of a stop bolt underneath. – Daniel R Hicks Oct 20 '19 at 18:50
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What you have is an adjustable stem. The headset is the pair of bearings in the head tube that the fork steerer tube runs in, that allows the handlebars and fork to turn.

You are tightening the correct bolt, however, check that the nut on the other side is captive and not turning when you tighten the bolt. If it is not captive you need to hold the nut with another wrench as you tighten the bolt.

These types of adjustable stems usually have a a series of radial splines on the inner and outer parts that lock into each other when the bolt is done up. The splines - being made of relatively soft aluminum alloy - get worn down and deformed, and can no longer hold the forward part of the stem in place. If yours is repeatedly slipping the splines are probably very worn.

The solution is to buy a new stem. You can either find a replacement adjustable one, or knowing where you like the bars to be get a fixed one that replicates that position.

Replacing the stem is relatively easy and videos on how to do it are easily found. The tricky bit is preloading the headset bearings. You have to get this right or the bearings will be damaged when you ride, or in the worst case the bike will be dangerous to ride.

You also need to replace the steerer tube top cap. This is necessary to set the bearing preload. Even if you don't replace the stem the lack of top cap is allow water to enter the top of the steerer tube and accelerate corrosion.

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  • Agreed. I'd also suggest replacing that rusty star-nut. – Adam Rice Oct 19 '19 at 17:15
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    I'd just clean it up with some wire wool and WD-40. Star nuts are a total pain to get out, and it's also hard to install a new one square if you don't have a proper tool. – Argenti Apparatus Oct 19 '19 at 18:12
  • Agreed - these are ideal for figuring out what angle and length you really want, and that's all. – Criggie♦ Oct 20 '19 at 3:03
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I concur with the other answer m; I think if you were to disassemble this stem by taking the bolt out totally and have a look at the surfaces that get clamped together you'll see that instead of having ridge that align and grip each other like cog teeth, they have been worn smooth and can no longer grip each other effectively

If you're after a temporary repair you can use a small drill to drill one or two holes right the way through the stem, parallel with the bolt you're tightening, and then insert something (screw, bolt, chopped off bit of spoke etc that is the same diameter as the drill. This will prevent the stem from moving but bear in mind that drilling it will also weaken it so if you're planning on using this bike for hardcore downhill or stunts at the local skate park, replace the stem with a fixed one (they're cheap). If you're only commuting on it and maybe bumping up and down the occasional sidewalk it'll be fine. Analyse your riding style and see if you're riding with too much weight on the handlebars too, and if you replace this stem with another adjustable one, stay on top of the adjustment and keep the adjuster bolt tight; when it's loose it will cause a slipping which will wear out the teeth that stop it from rotating - in summary it's loose and doesn't grip because it spent too long in a too-loose state, chewing itself to bits

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