My 10-speed Shimano Dura Ace "aero" shifter was getting stuck on the large cassette cog. When I swung the shift lever that is closer to the handlebar, nothing at all would happen. Occasionally it would work for a few shifts but then revert to the failure mode. Lubrication did not help. Actually I've had this happen on two different shifters, the other one Ultegra, which appears to be identical.
I decided to take the shifter apart to see what was happening. In both cases, a tiny spring was broken and causing the problem.
This video tells how to see if the spring is broken without taking the shifter apart:
https://www.youtube.com/watch?v=-1r9CESluVQ
https://www.youtube.com/watch?v=-1r9CESluVQ
Here's how I replaced the spring.
The older non-aero shifters were close to impossible to disassemble, but the aero shifters, with the shift cable under the bar tape, are much easier, though they are still difficult. This YouTube video shows how to take one apart.
You will see the problematic spring at 2:40 in the video. It's one of the
first parts that come out, so you can skip the following steps.
The broken "spiral" spring is made of 0.025 square wire. I found a coil
spring with round wire about that size, straightened it out, and bent it
into a spiral. I did not need to anneal the wire to bend it. Perhaps some
springs would require annealing and then properly re-hardening them.
When I first tried the spring it was not working smoothly. I had to straighten the spring so the spiral was very flat. Otherwise it was dragging where it was "pinched" by the mechanism.
A few notes on things that are not clear from the video:
1) At 1:18 a 1.5 mm hex wrench is used to loosen a set screw that
locks the main shaft. It is not explained in the video.
2) Shimano tool Y6RT6800 can be used to remove the C-clip from the brake lever pivot. (http://si.shimano.com/pdfs/dm/DM-ST0002-05-ENG.pdf )
2) Shimano tool Y6RT6800 can be used to remove the C-clip from the brake lever pivot. (http://si.shimano.com/pdfs/dm/DM-ST0002-05-ENG.pdf )
3) Note the exact order of the pieces as you slide off the main shaft
4) When you are replacing the two parts that have swiveling pawls on them, make sure the pawls are properly positioned. Once the piece is on the main shaft the pawls cannot be adjusted.
5) When you install the brake lever, make sure the shifting lever is placed into its hole before you press in the pivot pin. (9:40 in video)
