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The large chainring on my mountain bike is missing a number of teeth on one side (apparently the side I like to go over rocks on). Is it harmful to the drivetrain to ride without these if I'm not having issues loosing the chain? At what point should i replace the chain ring?

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If you use the large ring at all I'd say you should replace it when there are two adjacent teeth missing. And with missing teeth you need to replace the chain a hair earlier than normal. – Daniel R Hicks Oct 8 '13 at 7:25
You may want to get a bash guard to protect your big ring if you're loosing teeth. If you do not use your big ring much, then just put a bash guard in it's place. – Benzo Oct 8 '13 at 15:45
@Benzo, if OP is not using his big chainring, and it is taking the hit's isn't it's function already a lightweight bash guard? – Vorac Nov 18 '13 at 12:15
A real bash guard is going to be stronger than an aluminum chainring, and you are less likely to find its teeth embedded in your calf. – Benzo Nov 18 '13 at 18:30

While replacing any part comes down to when you want/can do it, you should replace any damaged part immediately. That's true whether you experience symptoms of failure or not.

Missing teeth on a chainring may not seem like a big deal until another one breaks off, causes chain slip or a dropped chain that sends you over the bars and into a tree or rock. Suddenly a missing tooth seems like a big deal.

As a side note, you should probably invest in a bashguard/ring if your knocking teeth off your chainrings!

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And invest in a helmet with face shield if you're knocking teeth off elsewhere. – Daniel R Hicks Oct 8 '13 at 20:06

Most non-pro riders will replace the entire drive train of chain, chain rings and cluster at once because of wear-in I would wait until you do that change over.

My cycle on this is usually 18 months or so. YMMV depending on the muckiness where you ride and how many km you ride.

On the other hand if you start getting chain-slip when using the ring then its probably time to do it.

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