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I've removed my rear derailleur so many times that I think I messed up the hanger threading this latest time I reinstalled it. It's very hard to screw in, doesn't even go in straight, and now not even my hanger alignment tool screws in properly. Since my bike is so old, I just bent the hanger to account for the miffed threads and reattached my derailleur the best I could as a temporary fix. Now I want to fix the threads. I've since learned of some methods involving tapping the threads as well as drop-out savers:

How To tap and Clean Threads On Your Derailleur Hanger

How To Fix Derailleur Hanger With Stripped Threads

Based on my description, is it likely that I cross-threaded the hanger? If I need to resort to tapping, how do I determine the proper tool to use? Are there bike-specific tools or will pretty much any tapping tool work so long as the measurements are correct?

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If its a replaceable hanger, simplest option is replace it.

If not you might be able to repair it with a 10mmx1mm tap. If that does not work, a Helicoil type repair is possible. Its one of those tasks if you are not 100% confident you can do right first time you might be better off handling over to a shop (An engineering shop or well equipped bike shop) as it is possible to make it worse.

Refer Park Tool Basic Thread Concepts

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    Another dirty hack is to try installing the hanger bolt from the inside. This can work like a tap but doesn't cut so much as reform the threads. Same trick works on pedal threads. – Criggie Oct 1 at 22:14
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    The hanger is built into the frame. Looking for a thread tap now. Some of these are right- and/or left-handed. Which do I choose? Edit: Seems that righty tighty is right-handed, so I think I need a right-handed tap – oscilatingcretin Oct 1 at 23:44
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    As @ Criggie has suggested try screwing it in from the wheelside, if you can then get the threads from the correct side started, apply some thread locker to strengthen the missing/ damaged threads. – mikes Oct 1 at 23:49
  • @oscillatingcretin You do need a right handed tap. 10 mm x 1 mm (1 mm is the thread pitch). Park Tool's Tap10 is an example. – Jeff Oct 2 at 8:42
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If the hanger is a separate piece the proper tool to use is a personal computer - to get on the Internet and order a new hanger. Separate hangers are wear items that are designed to be replaced. I think you mean you have a hanger is integral to the frame though.

If the thread is lightly damaged you can run a regular thread tap through it. This will remove the damaged thread areas, so teh majority of the thread area needs to be OK.

If the thread is more damaged there are special taps for repairing threads that roll the threads back into place rather than re-cutting them. I believe they are designed for steel and don't work well on aluminum alloy though.

If the thread is truly messed up it will have to be drilled out and an insert put in.

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