Sorry this sat out here so long without a reply. Sometimes if you post with a little more detail it gets people to bite faster. To be honest, I'm not 100% sure I'm answering this correctly - are you talking about your chainrings?
If so, (and I got this tip from Zinn and the Art of Mountain Bike Maintenance), the best way to do this is to take an adjustable wrench and use it to bend the ring back into place. If it's your middle or inner ring, you're going to want to take the crank off, but your outer ring you can probably do while it's still on the bike. Just clamp the wrench down on the ring and slowly bend it back into place. If it's a big bend, bend it a little bit at a time in many different spots, because Aluminum likes to snap when you work it too fast. I've done this for small bends and it works great.
You're talking about your cassette, not your chainring. Sorry. Here goes:
You can't repair it per se, because the small cogs are too small to work back into shape and the big cogs are permanently atached to the pider/carrier. When you say second from the top, do you mean the second smallest or the second biggest? The reason I ask is because if it's the second biggest (like a 26 tooth) then you need to buy a new cassette, unless you can find a way to get the cog off and bend it back into shape. If it's the second smallest, like a 13 tooth, you can just buy that one cog to replace the one that's bent.
I hope that answers your question, and sorry for the misunderstanding (one of my biggest pet peeves is when people respond to postings without reading the question thoroughly, and here I've gone and done it myself!!)
I've had a bent larger cog on my xt cassette before, I had just bought it and didn't want to replace it so since it was scrap anyways I used a punch and hammer to bend it back and voila, I got it back to staight again. There are holes in the cassette that make it easy to put a punch in to prevent it from slipping.
I was surprised at how hard I had to hit it though, work up to it, you don't want to bend it the other way.
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