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My flagship BSO has a threaded headset.

I recently discovered, to my surprise, that when I twisted the handlebars hard while restraining the wheel, I could move the stem. It isn't nearly loose enough to move while I'm riding, so I guess it's not a problem.

The top bolt was tightened as hard as I could get it with one of those little bike tool hex sets (lever arm about 3.5 inches long).

Are all threaded headsets like this, or is mine wonky?

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    It is not necessary that the quill be so tight that is cannot be twisted at all, but it should be tight enough that there is no danger that it will twist when you hit a bump or otherwise stress the connection. – Daniel R Hicks Oct 4 '15 at 1:37
  • If you have expander stem, this is kind of normal. – Rilakkuma Oct 5 '15 at 14:09
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    Highly recommended to get the stem out and check it's bottom surface near the wedge. Sometimes the wedge is tighten in bad position, and damages the quill. The surface must be smooth. If it is damaged, next time you tighten the stem, wedge will go again the wrong way until the quill will break. – Alexander Oct 6 '15 at 10:14
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Note that the large threaded locknut and top race on the threaded part of the headset has nothing to do with the connection with the handlebar, which uses a stem quill into the steerer tube (See the Sheldon's site for details). You tighten up the top race/locknut only if you feel "looseness" between the handlebars/fork and the frame.

Picture of a headset with labels

If you can move your handlebars separately from your fork, you have to tighten up your stem quill more using that little hex bolt. Sheldon again has a wonderful diagram of the wedge bolt.

Sheldon's picture of a stem's wedge bolt

An allen key may not be giving you enough torque -- you may want to use a hex bit on a socket wrench or torque wrench. You'd have to give us the specs on the hex bolt for us to look up the appropriate amount of torque -- but it's certainly more than you could get with an allen key. But be careful of using a socket wrench as that may give you so much torque that you snap the bolt or split the steerer tube -- the use of a torque wrench is strongly recommended.

It sometimes helps if you can take the quill stem out, use alcohol to remove any residual oil off of the wedge. You can also rough up the outside of wedge a bit with a file. Some people also use coarse grit (lapping compound) or carbon-fiber assembly paste to increase the friction and reduce galling risk.

You may want to grease the hole at the top of the stem quill where the wedge bolt enters as well as the surfaces between the quill and the wedge to reduce friction. If the bolt has a habit of loosening itself, you could use blue Loctite on the bolt threads.

But the main problem is that with BSOs, the tolerances are so poor that something is either too tight and galled into place, or so sloppy that no end of tightening will get you solid tight without breaking something else. And sometimes the drones at the factory do something truly idiotic like put a 25.4mm stem into a 26.00 steerer tube.

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    Do note that, especially on an old, rusted bike, it's possible to tighten the binder bolt so tightly that the steering tube splits open. – Daniel R Hicks Oct 4 '15 at 12:42
  • Thanks @DanielRHicks - I'll note that along with the bolt snapping. – RoboKaren Oct 4 '15 at 19:29
  • A proper, long allen key should be quite strong enough. Some grease (or at least WD40) on the wedge’s sliding surfaces might help, I’ve had stems where it was quite corroded. – Michael Oct 4 '15 at 19:41
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    Being sort of opposed to using alcohol and using a lot of torque. Oil residue is important so that aluminium of the quill stem does not come in contact with the steel (if steel) steerer. And a lot of torque is simply not necessary because steerer does not experience any enormous forces. – Rilakkuma Oct 5 '15 at 14:06
  • I've add a note that the reason you might want to use lapping compound is to also reduce the risk of galling. – RoboKaren Oct 5 '15 at 14:58
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One thing to keep in mind is that "real bikes," and maybe some BSOs too, can have a butted steering tube – one where the wall thickness at the bottom of the tube is greater than it is further up the tube. This means that if the stem is inserted to far into the steering tube the area for the stem binder to grip will be limited and it will be very difficult to get the stem to grep solidly in the steering tube.

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