My son has a Boss Stealth mountain bike, The front derailleur has stopped working. It is a 3 cog Sunrun 42t (picture 1) which I am finding difficult to find.

The bike originally had Grip Shift changers and was changed to lever/brake combo about 6 weeks ago and worked fine so I don't think that has anything to do with the issue.

I have looked on Ebay and found so many different type I'm not sure which model I can replace it with. Picture 2 looks similar and has the correct teeth configuration. Picture 3 I'm not sure where the cable would attach.

Can anybody suggest a suitable replacement unit please

picture 1

picture 2

picture 3

  • 2
    The main issues are that the new unit have compatible mounting, and that it be "top pull" to match the existing unit. And that it be capable of spanning 3 chainrings. Jun 16, 2020 at 20:24
  • this is why i thought number 2 would be suitable Jun 16, 2020 at 20:28

1 Answer 1


I think probably a Shimano Tourney top-pull derailleur that can accommodate the chainring sizes you have will work. You can see the basic specifications here to help pick out the correct model.

Front derailleurs are very basic mechanically and don't 'stop working' unless they actually physically break. It's possible you simply have the derailleur badly adjusted, or there is something wrong with the cable or housing.

Inspect the cable run from the shifter to the derailleur and make sure the housing is properly in the shifter body and all frame stops.

Disconnect the cable, tension the cable manually and work the shifter. Check the cable pull in and let out smoothly and does not hang up in the housing.

Verify the derailleur will swing from the smallest to largest chainring (it helps if you can pop the chain out of the cage).

If all that checks out, follow a front derailleur adjustment procedure. Park Tool has a good page and video with step-by-step guidance.

  • the derailleur wont move when we try to move by hand with no cable attached Jun 16, 2020 at 20:25
  • @JohnSayers that sounds like its wedged or damaged. Did you have a collision? Has someone kicked it? The front mech should move if you pull it to the right. "Top pull" refers to which way the wire goes, the opposite being bottom pull where the cable pulls down to move to a bigger chainring.
    – Criggie
    Jun 16, 2020 at 20:33
  • it is a top pull, he said it hasn't been damaged but i have my suspicions Jun 16, 2020 at 20:47

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