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The medium cage (GS) on the 12speed XTR model is supposed to be used only with a 10-45 cassette on a 1x setup, as the max sprocket size is 45t and total capacity is 35t.
If you have a 10-51 cassette and/or a 2x setup you will need a model with a long cage (SGS).
Thanks, I have a 10-51 and my current M8100 is SGS. Thanks Aglo.

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XC iconoclast
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This is what can be so strange about some Shimano terminology: Deore 5100 11-speed has a greater max cog capability than SLX 7000 or XT 8000. Because the "x100" series came out recently with 10-51 12-sp or 11-51 11-sp; older SLX and XT "x000" are 11-46 sp. I completely forgot about the new 11 speed groupset because everyone was so focused on 12-speed 10-51t. I and I'm sure many others assume SLX and XT SGS would have greater range than a Deore 5xxx series. Wrong! I found out today (and remembered after looking it up) when the bike shop put on the XT 8000 rear derailleur during the build and told me there is one that can handle 11-52t, which is my cassette size. The M5100 is only $56 on Amazon, about 1/2 the price as XT, greater range. I'll try it later when the XT wears out, usually happens to me about 1000-1500 miles.


SHIMANO Deore RD-M5100-SGS Rear Derailleur - 11-Speed, Long Cage
 

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I screwed up, and bought a shimano xt 8120 sgs derailleur, which is ratted for a 10-45 tooth 2x drive train.

I have a 10-51 tooth 1x drive train.
I’m assuming this derailleur won’t work on a 51t cassette?
singletracks magazine said it will work on a 10-45t 1x cassette.

unfortunately can’t return it, as I bought it last fall, kicking myself now 😟
 

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I screwed up, and bought a shimano xt 8120 sgs derailleur, which is ratted for a 10-45 tooth 2x drive train.

I have a 10-51 tooth 1x drive train.
I’m assuming this derailleur won’t work on a 51t cassette?
singletracks magazine said it will work on a 10-45t 1x cassette.

unfortunately can’t return it, as I bought it last fall, kicking myself now 😟
It won't work, it has shorter cage that long version. You can change cassete to 10-45 and gain some strength with along the way :)
 
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