Yesterday I took the wheels off of my rain bike and put the good wheels from my Venge on it. I did this because the wheels on the Venge are lighter by almost a pound and they spin faster. Once the cassettes were swapped (the Venge has a 10 speed drivetrain and the Trek 5200 rain bike, a 9 speed), I had to dial in the shifting, to index the rear derailleur. I was just a little off and when I arrived at the meeting place I tried to adjust it the rest of the way… I screwed up and worked it so far out that I de-threaded it. Oops. I lived with it where it was, but I’d lost one gear – the easiest (biggest) on the cassette. The ride, thankfully, was flat. Only 800 feet of climbing in 100 miles – there are literally no hills on the ride. I knew I wasn’t going to need the gear anyway.
Now, the photo above is after I’d fixed it. The shifting is perfectly dialed in and I’m exactly halfway between the limits of the barrel adjuster. Ten minutes before, the barrel adjuster was all the way screwed out of the threads of the derailleur. Oops.
Once the bolt is unscrewed from the derailleur, chances of getting threading it back in correctly is not advised. You’ve got an aluminum derailleur body and a steel (or worse, stainless steel or titanium bolt. Get it started wrong and you’ll end up needing to have your derailleur body tapped to fix the threads. Not exactly expensive, but a pain in the butt for sure).
The proper way to fix this is quite simple. Shift the rear derailleur to the smallest cog in the back. Loosen the bolt that holds the shifter cable to the derailleur. With the slack you can thread the barrel adjuster bolt in properly. Pull the slack out of the shifter cable, tighten the cable bolt, index the derailleur and Bob’s your uncle.
(The reason we shift to the smallest cog in the back is that’s the gear with the most slack on the cable)