Tech Slipped P-Tension Spring on Shimano Derailleur

GrantT

Maximum Pace
Oct 2, 2012
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Setagaya
#1
Back in October last year my 105 rear mech borked on a group ride at the top of Shiraishi. SomeOWENe diagnosed a broken B-axle spring (the spring in the part that screws onto the rear dropout). Looking at it when I got home I was a bit skeptical but ordered a new B-axle assembly right away and... a new rear mech. Why both? I have no idea. But the new part was an upgrade so I did the natural thing and quickly attached it to the bike and left my broken 105 and new B-axle assembly to gather dust in a corner of my shoebox apartment.

But the borked 105 rear mech would not be forgotten. It niggled at the edge of my awareness whispering "fix me" in the night like a sleep-talking Chris Martin, and this afternoon I actually took it apart. I took apart the B-tension and P-tension assemblies and found both screws seemingly intact. Then I put it back together but the cage still stuck out at a funny angle. On a whim I then did a search for "slipped P-tension spring" and found the video below (go to about 3:20 for the important part):
And now my 105 rear mech is working again.

So if your rear mech kind of stops working where the cage is sticking straight down (described at about 0:40 in the video) it should be fixable with simple tools. I reckon it can even be fixed without removing the chain. You need a 5 mm allen key to take off the rear mech, a 3 mm to remove the top jockey wheel, a 2 mm to remove the P-screw, and some kind of pliers to align up the holes on the outer plate so the spring end can fit in.

Edited because I can't name my parts right. Cage, not hanger/dropout.
 
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