Rebuilding the Herman Miller Aeron chair arm
The spring part in the arm is tricky to put back:
Don’t have any tension on the spring, attach to the chair and bolt on, then with pliers you can force the spring under the lip.
I have found that there is a trick to getting it back in without breaking anything. Try and partially reassemble the armrest similar to picture 4. Put the plastic indexer piece in the aluminum armrest frame and put the spring in so it is just resting against the indexer arms under no tension. Then quickly flip the whole assembly on the armrest support and put the rear bolt in and finger tighten it. Now the armrest should look similar to picture 4, just without the front bolt installed and the spring not under tension (not clipped under that lip you can see in the picture). Now, with a pair of needle nose pliers and a flat-head screwdriver you can force the spring back under that lip. This part takes some trial and error to find a method that works for you, but putting the armrest together like this allows you to man-handle the spring without the risk of breaking your printed indexer or having the spring go flying. I broke around 4 of these indexers before I figured this out.
r/hermanmiller
Source: https://old.reddit.com/r/hermanmiller/comments/mvfj91/anyone_who_has_any_experience_changing_the/