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To prevent stealing of the seat, I replaced my "fast lock" seat clamp with a model using a regular screw:

Seat clamp

The problem is that the screw keeps unscrewing, and I'm already tightening quite strong. I could screw even further but I'm afraid to break something. The screw is steel M5x12, the collar is aluminium, diameter 31.8mm. The bike frame and the seat stem are aluminium.

I thought of the following solutions:

  • Apply some thread lock
  • Use a larger, 2-screw clamp, like the followings models:

enter image description here enter image description here

Which solution would you recommend?


Update - adding a picture of my seat clamp:

enter image description here

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If a screw comes loose, in most cases it is caused by not being tight enough in the first place. It may be caused by a badly tapped bolthole where vibrations might loosen the screw which is quite rare. But I tend to believe that the screw wasn't tight enough from the beginning. (In 40+ years of cycling I've experienced one slipping seatpost because of bad sizing tolerances but never a self loosening seatcrew.)

Tightening 'quite strong' is probably not strong enough. A torque wrench, a useful tool for working on bikes, at the recommended 6Nm setting is best here. You may apply thread lock but use the sort that can be loosened. But normally it shouldn't be necessary.

Double collars may be useful with carbon frames and/or carbon seatposts if you want to apply the clamping force to a larger surface.

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  • I'd tend to believe it is due to a bad quality bolthole. I could tighten like a barbarous, but I don't want to risk damaging/breaking the frame because of such a screw issue. I had understood thread lock shouldn't theoretically be necessary, but maybe in my case a small drop could be a good temporary fix. I like the fact that double collars distribute pressure, and as I have to get another collar, I'll go for that. And some thread lock while I'm waiting for receiving the new collar. Do you agree? Aug 24 '19 at 20:16
  • @GrasDouble - Just be careful what version of thread lock you use. You don't want the permanent stuff. Aug 25 '19 at 0:12
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    Sure thing. A small drop of the usual blue, normal strength. Aug 25 '19 at 2:49
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Sounds like your thread is damaged. Keep over-tightening and it will strip.

At that point the only options are replace or repair the thread with a helicoil. I've done this on an unsourceable seat clamp, and it worked perfectly. This is also the only option if you have an integeated seatpost clamp (ie one that is physically part of your frame.)

Since this is a replacement part anyway - you could look for a better quality clamp with better tolerances.

Another thought - you could replace the bolt with a longer one, and fit a second nut on the outside. Risks catching your leg/clothing.

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  • Yes, I think the culprit is the bolthole. Guess I just got a bad product. I also thought of a longer bolt (the current one doesn't reach the end, like on my 2nd picture), but nevertheless the clamp has to be replaced. Just one question, I'm afraid to damage or even break the frame if I tighten too strong, for the time I'm awaiting the new items (about 2 weeks). Am I too fearful, or should I really use another clamp asap? Aug 25 '19 at 2:57
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    Its hard to ascern the material strength of a metal remotely. I'd keep a good eye on it if I were you, check before every ride, and avoid stressing it where possible. Any sudden change, stop riding immediately and check it.
    – Criggie
    Aug 25 '19 at 4:29

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