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It's been difficult to locate a new XTR M9100 or XT M8100 with the SGS (long) cage from the few reputable U.S. dealers I buy from where Shimano will honor their warranty. The GS cage versions are readily available. In the past, I know it's been possible to switch cage length on Shimano derailleurs. In fact the Dealers Manual for the above derailleurs has a section describing replacement of the plates that make up the cage but doesn't specifically state its possible to change between GS & SGS cages. The key is whether the main body of the derailleur is different between an SGS cage der and the GS version.

Looking at the Exploded Views of the M8100 GS & SGS versions--they are separate documents, which is different from EV's of older model ders that had one exploded view that showed both GS and SGS cage assemblies on one document, inferring the same derailleur body is used--it appears all part numbers are the same except for the differing cage plates. GS cage EV

Further, comparing the derailleur bodies of the SGS & GS versions, there doesn't appear to be a difference in them. Can anybody confirm they use the same main body for both cage lengths? compare der bodies

I have a source for replacement SGS cage plates for the M8100 and the GS versions of the entire derailleur are available and I wish to run a 1x system with a 10-51t rear cassette which requires the SGS version to be used, sums up my motivation to faff around with this, as the Brits might say.

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  • If I remember correctly, the SGS and GS versions are NOT cross-compatible. The GS is optimized for the 10-45t cassette, whereas the SGS is optimized for the 10-51t. The difference should be in the parallelogram slant angle (also the jockey wheel offset, but that's determined by the cage). I'd imagine it's possible to make it work, but it wouldn't be the "ideal" setup. Don't have evidence for this though--I'll write a proper answer if I find some sources.
    – MaplePanda
    Jan 8, 2023 at 3:25
  • This is what I've heard too: differences in the main body between SGS & GS spec'd ders. I've come across similar question to mine in different but less "serious" forums that caution about differences in the body. I also found it curious there are two EV's of the M8100 der on the Shimano site, one for GS cage, one for SGS cage. In the past the two cage lengths were shown on the same EV of a given RD.
    – Jeff
    Jan 8, 2023 at 5:27
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    @MaplePanda from my research, the parallelograms seem to be same for the M8100 GS and SGS (the SGS cage is not a longer GS cage, the offset of the jockey wheel is different too - in the 8000 series, only the length differs). But the parallelograms of the 8100 and 8120 (the 2x version, that takes a 10-45 cassette) rear derailleurs are different.
    – Rеnаud
    Feb 4 at 6:46
  • @Renaud Oh, good to know. Keeping up to date with every new tech release is getting more and more difficult.
    – MaplePanda
    Feb 4 at 7:09

2 Answers 2

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They're swappable - we came across the same question, checked the docs and confirm the body is the same. A friend of mine made the same change; but gone from SGS to GS.

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  • @ Baris Atasoy Do you have the link or reference to the docs you checked confirming the bodies are the same?
    – Jeff
    Jan 8, 2023 at 5:31
  • Shimano docs, shimano site. I dont collect them as I regularly visit and look something at their site. Jan 8, 2023 at 8:01
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In the meantime, Shimano released a 12-speed version of their Gravel groupset, the 1x version of it uses the MTB 12-speed cassettes (10-45 and 10-51).

Two interesting observations for this groupset:

  • Shimano itself indicates that cages can be swapped to avoid buying a new derailleur to change the cassette size.
  • The exploded view of the GRX 822 rear derailleur indicates that the part references for the inner and outer plates as the same for the GRX and XT rear derailleurs.

This does not of course guarantee that cage swapping is possible for XT rear derailleurs, but for me that would be a strong enough guarantee.

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    Excellent catch of the identical part numbers GRX and XT rear derailleur cages share. Seems obvious the GS and SGS cages are interchangeable on the RD-M8100 as well.
    – Jeff
    Feb 7 at 11:15
  • @Jeff I'm indeed cautious because "a worst case" would be that the GRX is an intermediate between two supposedly different parallelograms of the M8100 (a bit like some rear derailleurs that have an intermediate pull ratio between 10- and 11-speed to work with both). But that seems very unlikely.
    – Rеnаud
    Feb 7 at 11:30

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