I have a hardtail MTB and just replaced my chain, which I broke on the trail. I bought a KMC X9 chain with the quick link and installed it myself. Shifting works fine and everything is smooth...until I really hammer down on the pedals. In any situation when I'm really mashing down on the pedals, the chain will jump/skip and make a loud CLONK! sound. Any idea what could be causing this?

For information about the bike:
- Windsor Cliff 29er hard tail MTB
- SRAM X-7 9 speed F/R Deraillers
- Truvativ 5D triple crankset
- SRAM PC950 11-34T cassette

The bike is fairly new, bought in April 2010. I've put around 500 miles on it, almost all of those being pretty rough trail miles. The chainrings and cassette do not look worn upon inspection.

It's common for new chains to skip under load when combined with used cassettes. At only 500 miles I'd expect your cassette to be OK, but you never know. The easiest way to check is to try another wheel or cassette. If the chain runs OK with that, you know that you need to replace the cassette.

It's almost impossible to determine the wear age of a cassette by eye, but if you still have your old chain measure that for wear using a ruler. Since it's off the bike hang it on a nail and measure 4' using a steel rule, and measuring from the first link to the same place on a link 48" ( 96 links) out. I'd suspect that your cassette is toast if that distance is 48-1/2 inches or more. If it's nearer to 48-1/4 or less, your cassette should be fine, and you'll need to look elsewhere for the cause of your skipping.

BTW- if it turns out your chain was stretched beyond the limit in only 500 miles, consider chainging your chain lube (I have to say that) ans/or your chain maintenance practices. You should be able to do much better than that, and you prevent cassette wear by reducing chain wear and replacing the chain at 1/2% (1/16" over 12") stretch.

Unfortunately, I junked the old chain and no longer have it. I also don't have another 9 spd cassette I can try. I suppose if this is likely to be the culprit I can take this to a shop. Otherwise, I'll need to buy a chain tool, cassette tool, and a new cassette to test it out. Any other theories?

If you're planning to do lots of riding in the future, you'll need a new cassette soon enough anyway. You might as well buy it now, to go with your new chain and give it a try. Cassette tools are cheap enough and easy to use, and you can get by without a chain whip (if you want to save dough) by wrapping a rag or old belt around the cassette and holding it with a pair of pump pliers (Channelloks, by any other name).

If the new cassette solves the problem, that's it. If not, switch back, and save the new one for the future, and look for another cause, which could be one of many, but I started with chain wear because the problem started with the change.