Bicycle Mechanics - replacing crankset&chain&cassette

Hello,
My bike is 10 years old. Some teeth on the crankset are worn. I figure I can replace this and the chain as well. I don't know about my capabilities of replacing the cassette, though. Can I just replace the crankset and chain, or do I have to change the cassette, too? It's Shimano STX (3*7).
I'm sure this has been asked before, still I would be grateful for some advice, as the cassette replacement would probably mean for me and my abilities taking it to a repair shop (despite books and websites).
TIA, Chris

If you can swap a crankset, you can replace a cassette. There's actually a lot more room for error in the crankset installation than either the removal or replacement of the cassette. You simply need the proper tool for the cassette lockring and a chainwhip though a torque wrench can also come in handy. A torque wrench is a MUST for the crankset installation (in my opinion at least) so you'll need one anyway.

If your chain is worn enough that it has worn the chainrings too, your cassette is long gone.

www.parktool.com (http://www.parktool.com) has a great repair section that details out the procedure for both cassette and crankset removal and installation. If you still have questions after reading that, feel free to ask here.

Are you sure the chainrings are worn? Some teeth may look worn but are made that way intentionally to aid shifting. If ALL of the teeth are worn and pointed (like saw teeth) and the chain is skipping on the chainrings then they should be replaced. If most of the teeth are ok and the chain runs smoothly on them, the rings are good.

Chainrings are very long lasting unless they are subject to harsh and abrasive riding conditions. 30,000+ miles on a set of chainrings is common.

A worn chain will require a cassette replacement as rear cogs are both faster wearing and much less tolerant of chain wear than chainrings.

Measure your chain to see if it needs replacing. Pull all of the slack out of the chain and use a good steel ruler. A 24 pin interval of new chain will exactly 12 inches. When any 12 inch interval measures 12 1/16th" it should be replaced. If it measures more than 12 1/18th" the cassette should be replaced as well. Chainrings (cranksets) should last a long long time unless the teeth have been damaged. As already stated, chainring teeth are made with special and strange looking shapes to assist shifting.

Al

If the chainrings are riveted to the crankset new chainrings require a new crankset. Then consider buying one with bolted chainrings, which will give you the option of selecting different chanring sizes.

Hello,
Thanks for all the excellent and quick advice. This is a great site, one advantage of globilization, I guess (this being written from Germany). I think I did indeed, due to mishandling, actually manage to damage a couple of the chainring teeth and will have to replace it (as it skips sometimes under pressure), but I'll look into this further. Thanks again, Chris

it skips sometimes under pressure

Almost a sure fire sign that your cassette is worn out because your chain is worn past the serviceable limit. Replace them both and you should be good to go.