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I have a Shimano 11-42T 10s cassette and Shimano XT M786 SGS Deore RD, can I make these work together by using goatlink or am I wasting my time?

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    Why a goatlink? Did you encounter some problem or anything? It should just work as is. Are are you just seeking some improvement of the shifting with the goatlink?
    – Vladimir F
    Apr 29 at 11:12
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As correctly stated by @ Renaud, your derailleur has a max rear cog of 36 teeth. In practice, however, it can handle larger cogs in most situations with 42 teeth likely being the absolute most it can handle. The issue is getting enough space between the upper jockey wheel of the derailleur and the largest cogs so that the derailleur's jockey wheel doesn't interfere with the cassette cog. There should be 5 mm of space between these two {tip of tooth to tip of tooth) when the chain is on the smallest front chainwheel and largest rear cog. You may not achieve this amount of gap but as long as there is no interference between cog and pulley wheel, that'll have to be good. The gap is adjusted be manipulating the B-screw of the derailleur. To increase the gap the B screw gets turned in clockwise. Some folks put the B-screw in backwards to get a little more length. Substituting a longer screw can work too. IIRC, B screws are size M3.

Shimano Specs Technical document!

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  • Are you really sure SGS is 36 only? See my comment under the other answer.
    – Vladimir F
    Apr 29 at 15:25
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    @ Vladimir F Yes. I've added a screenshot of the specs from the Shimano technical documents website.
    – Jeff
    Apr 29 at 21:53
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The RD-M786-SGS is rated for a big sprocket of max 36T, so if you want to follow strictly Shimano specs, no.

That being said, Shimano has high standards and is very conservative, so it is possible that it works. I personally just installed on my bike a Microshift 9sp 11-42T cassette (no goatlink) on an Acera RD-M3000, rated for 36T max (I had to adjust the B-screw). If you want to do things properly, you may also need to add some links to the chain, especially in a 1x setup. I have a 2x (46/30), I've let the chain as is and try to avoid even more the big chainring/big sprocket combination — I was also fearing that adding some links would cause the chain to swing too much on rough surfaces.

It seems to work well so far, but I haven't tested extensively in rough terrains. That being said, I tried because I have found several comments of other persons saying they have installed successfully this cassette on derailleurs rated for 36T max (including some old Deore and XTs).

I would assume a similar reasoning is also valid in your case.

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  • Wait, I looked into an eshop and they say that SGS is up to 45 teeth (and GS up to 34). Are they wrong? kupkolo.cz/… One customer in the review says it works with 50 teeths.
    – Vladimir F
    Apr 29 at 15:24
  • I took the info from Shimano's website: bike.shimano.com/en-US/product/component/deorext-m780/… . For 50T, maybe Shimano is even more conservative that one can think ;)
    – Renaud
    Apr 29 at 15:43
  • It is strange that both GS and SGS would have the same max 36T and would only differ in the total capacity.
    – Vladimir F
    Apr 29 at 15:57
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    Why? Unless I'm wrong the upper part of the derailleur is the same, and it is this part needs to have the necessary range to position the cage relative to the cassette. The cage length shouldn't influence that. That being said, I sometimes have the impression that Shimano specs are also constrained by their cassettes/chainrings offer: the M786 seems to be in the trekking range, and in this range, cassettes are 11/36 max, with triple chainrings, so 36T max.
    – Renaud
    Apr 29 at 16:09

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