I had to pack my bike (our folding tandem to be precise) for airline travel. I read up on how to do this and there was the recommendation to remove the rear derailleur, which I found plausible.

Removing it and packing it in bubble wrap and tapping it to a safe location on the inside was all very easy.

When I tried to re-attach the derailleur however (at the beginning of vacations! On a Sunday!) the bolt did not catch the thread correctly and stopped. After a few tries I started seeing filings in the thread in the hanger, so I stopped. I had done this once before as a dress rehearsal with no problem, but now I have one. I'll have to take it to a mechanic tomorrow when the shops are open.

In the meantime I'd like to know what i did wrong. Any ideas? Part of the problem seems to me is the fact that it is very difficult to see the bolt being directed correctly when the thread starts catching because so many objects obstruct the view. Any tricks?

I am also disappointed that nobody in any web site I consulted put a warning about this into the bike-packing instructions. I start suspecting that taking the risk of damage to the derailleur during transport (doesn't happen unless very careless handling occurs) is maybe smarter than taking the derailleur off.

Many nuts and bolts one can start by hand without much risk of damaging threads. Unfortunately the rear derailleur may not be one of those.

Do you have an integrated or removable derailleur hanger?

The one thing I would say is to always be gentle and never force anything. Sometimes it helps to turn a screw backward a revolution or two before going forward, and maybe feel where the threads engage. Also, be careful of the B-Screw adjustment stop which can get in the way.

Starting a bolt and cross threading it does screw up the threads ..

Remove the wheel and take the RD off the chain , & remove the cable, Rear mech now loose .

Then maybe you can screw it in from the back ... straighten out the threads ,

then Try again putting it in more carefully in its normal position..

Seriously, if you can't hand-thread a derailleur bolt without cross-threading it, you really shouldn't be doing ANY work on a bike.

Uhh...you cross threaded the bolt? Second bit...you gotta be kidding

+1 on this and next time loosening the cable will ease alignment issues....finger thread first and wrench after you are sure the alignment is correct.

I can understand how a der mounting bolt might get cross threaded, unlike some here. But I do agree that it's a sad case just the same. As FB mentions try running it (or a 10x1 bolt) in from behind to have the chance to reform the starting threads.

If you are not comfy doing this go to your LBS.

It would be nice to have more info. Like is the hanger replaceable? Is it AL?

As to the trick to get the der in straight- It's about taking care to visually align the bolt and hanger then only turning the bolt with enough force to feel the threads just grab and also have no "friction" indicating they're crossed. Another words it's about skill and touch. Using a screwdriver like hex wrench helps to get the visual alignment right, a "Y" or "L" wrench are hard to align as easily (and a multy tool sucks). Andy.

You are not the only one who has had this problem and should the threads have been severely damaged there is a solution: Dropout Saver

And I agree with Andrew Stewart, multi-tools really suck.

This trick has served me well.

Yes, I am never again going to remove a derailleur myself. though I do believe this could happen to anybody. I have never cross threaded a bolt on a bike before, but this one is just much harder to judge visually. It would be nice to have some sort of guide piece that keeps the RD perpendicular to the hole before threading.

To answer questions that came up: Finger threading is of course impossilbe; I used a hex wrench (standard with a 90 degree bend in it. I stopped turning as soon as I got resistance and forced nothing. Yet at the end after may be a dozen attempts I saw the thread damaged (not destroyed all the way through). Interestingly the bolt appears to be utterly undamaged, at least to the naked eye. Having a bolt that much harder than the thread is of course a recipe for disaster. The hanger is part of the frame, not detachable. Otherwise the problem would be smaller I guess.

This thread is an awful lot easier to cross thread than the others on the bike. People giving advice to amateurs really ought to put in a warning.

I teach the kids to do what you suggest. Helps a lot. Feel for that little thud when going backward, and then begin forward rotation.

[MENTION=357263]dblA[/MENTION], don't be too hard on yourself. Most of us have probably cross-threaded something at one time or another. The cable tension from the still-attached shifter cable probably didn't help matters. From what you say in your post, I'm surmising that no real harm has been done. Hopefully that is what the shop will find when you take in the bike.

It helps to learn to install bolts by "feel". Most mechanics would have no trouble threading just about any bolt blindfolded. For auto mechanics this is practically a job requirement.

On the RD, you want to use either a T-handle allen key or, if you're trying to save packing space, a traditional L-shaped key works too. Don't use a multi-tool -- they're intended for emergency use, not routine (dis)assembly. Put the long end of the key into the bolt head and hold the key close to the bolt head between your thumb and forefinger. Do not touch the T-handle or past the "L". Use a "rubbing your finger" motion to rotate the key just as you would if you could get your fingers directly on the bolt. Rotate the bolt a complete turn and if you don't feel it catch then adjust the angle slightly and repeat. You'll know by feel when it starts threading in correctly and you should be able to get two or three full turns into the threads before you need more torque than just your fingertips.

For really stubborn bolts, the "rotate backward trick" can be pretty helpful. You apply a little "downward" pressure on the bolt and as it rotates backward past the thread engagement point you'll feel it drop down a fraction of an inch. Stop immediately and rotate forward and the threads should engage.

Don't forget grease on the bolt. Steel and aluminum need something between them.

My job in jewelry stores requires me to change batteries in watches. Imagine the cross threading scenario with GOLD screws! They will strip with almost no effort. The "reverse turn 'til you sense a click" is the only consistent technique which seems to work. I use this technique in almost every situation where threaded parts are used.

I brought in the bike this morning. The store owner made a long face--I don't know him, I am in France on vacation--and explained to me that the thread was destroyed--luckily I had looked up the word before going there. Eventually he said it could be fixed by tomorrow, maybe he had to use some special parts, maybe not. He all but retracted his initial diagnosis by the time I left.

BTW it is a steel frame, not aluminum.

OP....hope you enjoy your trip....but if still problems when you get back...... Silva Cycles is close to you.....

Always do this if you lack experience. Especially on bottom bracket threads!

These are "fine" threads so very easy to dmage.

I destroyed the bottom bracket threads on a project when I was new to the hobby

You mean something like "If you are unable to walk and chew gum at the same time, do not attempt this repair"?

Oh, the worst possible things always happen away from home. On a recent trip, I got to the destination, and one pedal was nowhere to be found. Not expensive to fix, but frustrating nonetheless.

I'm surprised it is taking the mechanic a day to fix. Perhaps the issue is worse than it sounds. Enjoy your time touring locally before heading out.

The mechanic charged us 35 euros and had it all fixed. I think he was able to install it on what is left of the thread. It works just as fine as before.

I believe he was showing off when we first saw him. Competent craftsmen always show off. Before we took the bike away he wanted to see it folded and measured it folded and unfolded. He had never seen one of these KHS folding tandems and he was impressed obviously.

That's great news!

And a folding tandem is way cool. Just to see a tandem itself is an unusual thing. It's always fun to see a different style of bike than I'm accustomed to.

This.

Also, sometimes, you can put the screw in from the other side. That may straighten out the threads so you can get it in on the correct side.

On any bike frame, the bolt is always going to be harder than the frame material, even for a steel frame and even more obviously for aluminum or carbon. There have been aftermarket aluminum rear derailleur mounting bolts available in the past but those were made to break away in the event of an impact so as not to damage the frame. They were not intended to protect the hanger threads.

Now that all problems are resolved I want to thank everyone for their contributions and encouragement (and snark too!).