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I bought a 2nd hand read derailleur (Shimano M340) and now I've realized that pulleys wheel have not the metallic cover plates(washers) that Shimano derailleurs pulley wheels always have.

pulley wheel

What should i do? Using as it is? trying to buy this plates?

Is the diameter of these metallic plates compatible among Shimano pulley wheels? So, I would be able to took away from a broken or unused derailleur.

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In other Shimano groups, those washers are dust covers. They reduce the amount of dirt intruding into the bushing - that's the thing that the pulley wheel turns on. Bushings are cheaper alternatives to ball bearings. For use cases like pulley wheels, which are under very low load, they are perfectly sufficient. Dura Ace (and probably XTR) uses ball bearings. I've never had my hands on a pair of DA jockey wheels, but I assume the bearings would have a rubber seal, presumably in addition to the dust cover.

The RD-M340 is the Acera RD, which is a lower end group. I don't have my hands on an Acera RD, but in Googling for replacement jockey wheels, the stock Shimano units look like they have those dust covers. They're also very cheap, so if that dust cover is really missing, then I would replace the jockey wheels.

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  • Or get some pulley wheels with sealed bearings. The sounds of sand grains inside the bushings is unbearable. Feb 24, 2023 at 14:41
  • @VladimirFГероямслава is this a problem on components like Acera? I can't say I've noticed this with Ultegra, which has one bushing and one bearing. In any case, 3rd party wheels with bearings shouldn't be that hard to find. Shimano probably doesn't make these stock unless you want the DA or XTR wheels, which will be designed for much narrower chains.
    – Weiwen Ng
    Feb 25, 2023 at 23:10
  • Higher tier Shimano rear derailleur pulley wheels have sealed bearings. Essentially what you see after the metal covers are removed is a cartridge bearing. Same fundamental design as found in modern wheels, bottom bracket, headsets, and full-suspension frame linkages.
    – Jeff
    Feb 26, 2023 at 11:57
  • Actually, Ultegra is 1 bearing and 1 bushing, 105 is 2 bushings, and DA is 2 bearings. I don't know the configuration for the MTB groups.
    – Weiwen Ng
    Feb 26, 2023 at 15:35

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