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I am using a 10 gear XTR derailleur on a 9 gear cassette with a 9 gear chain and the tension pulley of the derailleur looks like needing replacement (despite teeth being very sharp it seems to work OK so far so i know i don't have to replace it, but i'm going on a tour so don't want problems during it). Should i replace the pulley with a recommended replacement (i.e. Y5XC98140) or use a pulley for a 9 gear XTR derailleur instead (i.e. Y5VW98120) since i am using a 9 gear chain anyway? The number of teeth is the same, so what's the difference between those pulleys? Can they have a different axle diameter as suggested here?

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    That's interesting, a 10 speed MTB derailleur should not work on a 9 speed system due to the different actuation ratio and cable pull between Shimano 9 and 10 speed MTB groups – Argenti Apparatus May 9 '19 at 12:54
  • I am using a continuous motion gear shifter SL-BS77, so it does work somehow, albeit not perfectly - i can't use all the gears (maybe i just didn't tinker long enough with it though). – InternazionalIV May 9 '19 at 13:27
  • I agree it shouldn't work but I can imagine with tuning it would work. If it were the other way around that would never work 9speed derailleur with 10 speed cassette. But as it is, a ten speed derailleur on a 9speed cassette would work imo. – David May 9 '19 at 19:40
  • Maybe the 9-speed pulley wheels have a different width or axle diameter -> I’d go with the recommended 10-speed replacement. – Michael May 13 '19 at 8:46
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Changing your pulley/jockey wheel to a higher/lower tooth count will not change the shift indexing of your derailleur, it'll only change the tension of the chain. (if you replace an 11 tooth jockey wheel with a 16 tooth, you might wanna add a few links on your chain)

Changing jockey wheels shouldn't matter for as long it has no conflict with the cage of the derailleur (the plate thing holding the jockey wheels), either being too large to fit or having incompatible axle-bearing contact/diameter (very rare though).

I'd recommend having narrow-wide jockey wheels especially if your derailleur doesn't have a clutch. There's a reason why SRAM adds it on their derailleurs and roadies/CX riders like it.

Also, no need for sealed-bearing wheels. Jockey wheels with bushings are sufficient especially if you plan to replace them every 6 months.

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