I wanted to replace a stem with a new one, so I first tried to screw the top cap off. What happened is that instead of the cap, the piece in the picture came out. What is the name? Is it bad that it came out?

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I managed to take the top cap out of it. Is it enough that I screw that piece back on the star nut?

1 Answer 1


What you have looks like upper part of an expander bolt. If the lower part is still in place, you can just screw it back. If not, you may have to fish the rest of the parts from the steerer tube and put it together before installing.

  • I managed to fish the rest from the steerer tube by flipping and shaking the bike. I put it together, but there is more play than before on the front wheel :/ What's the purpose of an expander bolt and when is it needed? Commented Aug 10, 2018 at 20:01
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    They are used with carbon fiber steerers to avoid scoring the inside of the tube with star nut.
    – ojs
    Commented Aug 10, 2018 at 20:05
  • @AlessandroCosentino the top cap bolt (with expander or star nut sitting in the steering tube) controls preload of headset's bearings. It should be used to control amount of play. Do not overtighten it however Commented Aug 10, 2018 at 20:06
  • @GrigoryRechistov Even after tightening a lot, I feel there is lot of play. Not sure everything is still ok with the expander bolt. Commented Aug 10, 2018 at 20:19
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    I figured out the issue with the front wheel play. The problem is that the stack of the new stem is 1mm higher than the old one (sigh...), so the top cap bolt wasn't engaging properly with the expander. I put back the old stem and the play isn't there anymore. Good news: no piece is broken. Bad news: I can't use the new stem until I get a 6mm spacer :/ Commented Aug 11, 2018 at 9:44

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