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I disassembled my rear derailleur to clean it; when I removed the sprockets they basically looked the same; both jet black and caked with all sorts of black grease-like-stuff. I figured they were the same so didn't pay attention. However, after a bit of cleaning they turned out to be different.

enter image description here

One has a metal bushing, says Centeron G-Pulley, and seems a bit beefier. Which is the top/guide/jockey pulley, which is the bottom/idler?

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From your photo, the left pulley (beefier, with metal bushing) is the top one, which goes closer to the cassette.

I know that because that's how things were in every shimano derailer I had over the years, and I think that makes pretty much sense, since the beefier pulley (the upper one) actually shifts the gears, while the bottom, thinner one is only an idler that keeps the chain tensioned.

As for the "centeron G" stuff, I have no idea what this means...

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By the way, "lucky you" to have an STX RC derailer, my wife's bike has one and it works great. It's been with me for years, I have beat the crap out of it in trails, it is even a bit twisted and has a lot of lateral play, and STILL it works great... – heltonbiker Apr 6 '13 at 23:44
I found this derailleur overhaul after some Googling, and it seems to confirm that the G-pulley goes on top. I'm guessing that G stands for guide. – Kibbee Apr 6 '13 at 23:44
The "Centeron" is about the top pulley deliberately having a little bit of side to side float: – armb May 8 '13 at 13:11

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