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I have a 32t chainring used for 9 speed and it has a few small signs of wear. Am I going to knacker my shiny new 10 speed chain if I use it? I'm also concerned that wear to the chain may cause premature wear to the new cassette also. Thanks.
Just reverse it on the spider to wear the trailing edges of the teeth.
If the chain ring teeth are pointy then replace. If the chain was changed at 0.75% wear (I recomend as chain checker like the KMC one) then the cassette will not need changing unless it is skipping. If the rings are worn then it probably worth replacing them. I have put new chains on old cassettes before with no problems at all so long as the chain is changed in time.
Definitely replace the chain ring. Tried to use an old chain ring with a new chain once and the chain would barely even bite.
Edit: Sorry, red wine kicking in.