Thanks for the replies and comments. I finally managed to compare with a new RD-M786 derailleur, and it's a medium size.

I think this is a Shimano XT 10 RD-M786 rear derailleur

  1. but is it a medium cage GS
  2. or a long cage SGS?

I took some measurements

  1. The cage plate is about 12cm to 14cm (4.72in to 5.51in) long
  2. The pulley wheels have diameter 4.0cm to 4.5cm (1.57in to 1.77in)
  3. There is enough space between the 2 pulleys to fit another pulley and some extra room left.
  4. The distance between the pulley wheel centers is about 10cm (3.94in)
  5. The ruler in the pictures is 15cm (5.9in)
  6. The bicycle wheel is 26in

According to this question How to calculate the capacity of a rear derailleur it should be a long (although my max cog exceeds 34)

  1. max chainring difference = 40 - 22 = 18
  2. max max cassette cog difference = 36 - 11 = 25
  3. total capacity = 43

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  • Looks like medium at first glance. The medium cage you could fit max one pully between the top and bottom, long you could fit 2. That lower pulley needs a replacement. It should have teeth the same as the upper.
    – shox
    Apr 20, 2023 at 17:39
  • Doesn't the backside of the hanger have Japan stamped in it with either gh or sgs beside it?
    – shox
    Apr 24, 2023 at 22:08

1 Answer 1


It is a GS.

By looking at the exploded view and the stock pictures, I see that the inner plate of the SGS cage has a hole in it that the GS does not have.

RD-M786 EV:

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