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Background: I have a full suspension mountain bike with SRAM GX Eagle components. I snapped my chain recently, and replaced it with a new one of the same model. Made sure to size it the same as the previous one.

After installing the new chain, I am now getting a slight skip on the upper pulley of the rear derailleur. Bike is rideable but want to fix this before I cause more damage. It looks like the chain is not completely aligned with the pulley almost. I haven’t looked at one in detail in great working condition so hard to compare. The teeth of the pulley are a little worn, so I’m thinking of replacing those. Not sure if I need to replace the cage as well, is it easy to tell if it’s bent? Also not sure how to check if the hanger is in good condition either.

Also additional note that might help, my chainring is also quite worn, could that have to do with it? Was not happening with the old chain, which had stretched quite a bit (don’t have exact measurements)

What should I be checking/doing and in what order? Pretty new to bike maintenance and want to learn from some experts! Thank you in advance.

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    Is the chain properly routed through the derailleur? There’s no real reason I can imagine for the guide pulley to be skipping since it doesn’t see much load at all. I know the tension pulley is a narrow-wide design which can cause skipping if misaligned, but that’s not the upper pulley. – MaplePanda Dec 22 '20 at 6:44
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    Check the chain for a stiff link! It may happen even with chains closed with a quick link/missing link when one of the other links is stiff from the factory. – Carel Dec 22 '20 at 8:13
  • Have you adjusted cable tension for good shifting performance? Is the derailleur aligned with the cogs when in a gear and shifting properly? – Michael Dec 22 '20 at 17:26
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New chains are not sized out of the box (i.e. they are usually several links too long). Part of installing a new chain is to size it by removing links prior to installing it to match the length of the old chain.

I suspect you may have mis-sized the chain. Otherwise it is possible that your jockey wheel on the derailleur is worn. Those can also be replaced individually.

I am not expert on sizing a chain, but Calvin is so here you go:

Also, no shame in going to a LBS to have them look at the drivetrain, size the chain, adjust the B screw as they do it day in and day out plus it requires a few special tools. You probably could get all of that fixed up for $20-$30 plus parts if any are required.

Hope that helps.

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    Jockey wheels can be used until they are very heavily worn (to the point where whole teeth are gone). They don’t transmit any power/force, they merely guide the chain. As long as the chain doesn’t slip of sideways (e.g. because the teeth are worn down to little stubs) everything should be fine. – Michael Dec 22 '20 at 17:25
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    @Michael: Yes, with a caveat though. It isn't usually a problem with 7-9-speed systems, but 10 or 11-speed drives may be more sensitive to worn, especially upper jockey wheels because they rely on a more precise chain guidance. – Carel Dec 23 '20 at 16:23

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