Man, ... today I found out what was wrong with a Sh. Acera derailleur that I took apart and modded back in late April or early May. I had drilled a lower tension hole for the B spring in its B axle bushing casing, to get the jockey to track a road cassette better, which it does. But the thing continued to be plagued by shifting issues. The B axle assembly seemed a bit wobbly.
Today I popped the thing off the thing again, and ... oops! I realized that the C clamp (which is dished for a bit of spring tension) was backwards, concave side in! Flip the clamp over mount, reattach derailleur: less free play evident! Go for a ride: almost perfect shifting, doh!
I must admit, if I don't know something I figure either (a) someone else does, so let them answer it, or (b) nobody does, Either way, I'm not bothered enough to research stuff
Is there a badge for "longest question"? That would be more of a challenge!