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Bike was in the shop recently and they said they needed to repair the derailleur hanger threads, which is integrated with the frame. They said they would repair the threads with a helicoil.

When I picked it up, they had drilled it out and installed a t-nut instead. This nut protrudes out the inside of the derailleur hanger and there is not enough clearance with the chain when in the highest gear, so the limit of the derailleur is set so you can't use the highest gear anymore.

Mainly my question is, can this be fixed somehow?

But also, why would a shop opt for the t-nut over just repairing the threads? This seems to have irreversibly damaged the frame.

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    It really depends on which threads needed repaired. Pic or 2 might help. Assuming the is the threads where the derailer gets attached to the hanger, then the quick fix would be to just replace the hanger, guessing though the shop would have done that as the first course of action if that was the case though.
    – Hursey
    Commented Aug 14, 2023 at 0:03
  • @Hursey the question says it's an integrated hanger, so on the face of it this is some very bad workmanship
    – Chris H
    Commented Aug 14, 2023 at 12:07
  • What's the frame material? I've only dealt with integrated hangers on steel, which has more options for repair than other materials. What type (and what sort of value) is the bike? This would be a reasonable fix - with permission - on something basic, but not on something of any real value
    – Chris H
    Commented Aug 14, 2023 at 12:10
  • I assume this can be fixed, probably by switching out the LBS that bodged that together in the first place. I'm always surprised that some don't even find the time to shift through all the gears or do a 2-minute test ride when messing around with the drivetrain and then hand bikes back to customers like that ...
    – DoNuT
    Commented Aug 14, 2023 at 15:39
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    There's enough information here, this is a fine question. The shop used an t-nut type insert instead of a helicoil, and now the wrench flat area of the t-nut are interfering with the chain while in the small cog. No need to close this.
    – Nathan Knutson
    Commented Aug 16, 2023 at 22:37

3 Answers 3

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This is likely just a mistake on the shop's part. Using a t-nut (they go by some other names) to repair hanger threads used to be very common. They're faster, although that's about the only advantage. I don't know that they fell out of favor because of interference issues like you describe, but it is certainly a possibility with them on modern bikes.

One way a shop could have gotten to this plan is simply not having an M10x1 helicoil kit. Another is if the damage to the threads was larger than the tap in the M10 kit (11.4mm major diameter according to this, I don't have one in front of me), but the nut type approach was able to work since you use a 12mm drill bit to install them.

If the 11.4mm number is correct and you have a 12mm hole in the hanger now, then it is impossible to simply switch to a coil insert. What may be possible is switching to a solid-wall insert such as EZ-Lok, which require a larger hole (the special tap for them is larger diameter) than coil inserts. There are a few different lines out there of products like this. The constraining factor for you in this case is how much material is left in the hanger to cut the threads for the solid-wall insert. If this is a steel bike and you're already at 12mm, there may not be enough left. Aluminum bikes with integral hangers often were built chunkier there, so you could be in luck. If you go with a solid-wall insert, its depth (thickness) will likely need to be modified to match that of the hanger exactly, so that you don't wind up with the same problem again.

On some bikes it could work in your situation to simply relieve some material off the inside of the hanger to make room for the back of the nutted insert. This would mostly be if it was an early aluminum bike and the hanger was very thick. There would likely be no way of making this work on a steel bike.

If it's steel, there are various hot repairs that would fix everything (new hanger, new dropout, build up the area with brass and retap it, braze in a piece of nutted insert, etc), but this is a hassle. Since the strength requirements are fairly low, it could also work to take the insert, cut off the wrench flats, and epoxy it in. Done right I see no problem with that.

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I have the same problem. I have an old Klein Stage Comp aluminum frame with derailleur hanger as part of the frame. I got it fixed in the same way as you and lost my highest gear in the process. Unfortunately I don’t think there is anything you can do about this because this fix requires a nut on the inside of the hanger. In the end I figured it was a small price to pay for saving my frame and better to lose the highest gear than the lowest. I just adjusted my shifter so I can’t shift into the highest gear. I think there are other ways of doing this repair that might allow you to keep you highest gear. I recall seeing something on YouTube. You can see the lack of clearance on these two photos. enter image description here

enter image description here

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may be a "Range Extender for Rear Derailleurs" could help. (like for example Wolf Tooth GoatLink or Wolf Tooth RoadLink)

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    If the problem really is as described, I can't see that helping. It won't move the T nut or the chain. But perhaps when the OP comes back with some pictures, I could be proved wrong
    – Chris H
    Commented Aug 14, 2023 at 12:08

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