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I've got a trusty Marin East Peak XC from 2008, but the rear derailleur's adjustment screws have all sheared and I need to replace it. The derailleur is a Shimano Deore LX RD-M581, but I can't find any exact replacements available.

How can I find a derailleur that will work?

I've found this extra info for my bike:

  • Rear cogs: 9 speed, 11-32

  • Chainring: 44/32/22

  • So the total capacity required is 43

  • I'm not quite sure if the current is GS or SGS

I've seen some advice that I can could get the right modern Shimano derailleur with 7 or 8 speeds, like an Alvio RD-M4000? I'm not particularly fussed about ultra-high quality or anything, I just need something that'll work well. I'm also a student though and can't really afford to screw it up and buy multiple versions.

Current derailleur (not pretty I know, its a little goosed)

  • You were correct about the required total capacity – Argenti Apparatus May 7 at 19:20
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If you wanted to stay with Deore, I still see M591 and M592 9-speed Deore rear derailleurs in the shops (a direct successor to your M581). Those are older series, but will be Deore-level. You want the SGS version. Even my old M772 Shadow XT appears to be available. That works to me very well.

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You can in fact use any modern Shimano derailleur for 7, 8 or 9 speeds that meets the total capacity and largest sprocket requirements. The reason is that Shimano used the same cable actuation ratio (ratio of cable length pulled to lateral cage movement) for MTB groups below 10 speeds.

If you want a derailleur that works as well as possible, get the current highest level 9 speed option. That would be the Alivio RD-M4000 which accommodates 32 to 36t largest sprocket and total capacity of 45. Rear derailleur specs are here: https://productinfo.shimano.com/#/spec/MTB/Rear%20Derailleur. You can look up various options.

Note: when I accessed that page there appeared to be many errors, so Shimano might be in the process of an update. Correct info can be found in the archive PDF files here: https://productinfo.shimano.com/#/archive.

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  • Thank you very much! I'll see what I can do :) – Matt Boland May 7 at 20:02
  • Calling Alivio the highest level 9 speed option might be a little confusing to some readers. Maybe "highest 9 speed option currently available at retailers" would be better? – user430 May 7 at 21:48

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