Bicycle Mechanics - chain stretch

The mechanic at my LBS says I have "Quite a bit" of chain stretch.

I know this messes up the cassette, but will it mess up other (more important and expensive) items like the chainrings or derailleur?

Thanks

Edit: I guess the derailleur shouldn't be affected since it is only the replaceable cogs that come into contact with the chain.

If the chain is stretched you need to, at the least, replace the cassette. The chain and cassette wear together. If the chain is as worn as you intimate chage the cassette at the same time.

If the chain is stretched you need to, at the least, replace the cassette.

Not necessarily. The chain could be at the replacement point but it might not have been used enough to wear in teh cassette.

It's not a problem at any rate, if you put a new chain on correctly and it starts skipping off the rear cogs then replace those too. If they don't... well don't replace them.

I'd swear that chainrings on my MTB wore as fast as the cassette; given the harsh riding and similar teeth count, it makes sense. I mean, I tended to ride slow, in lower gears, so I'd be worried that other riders who did the same might wear both cassette and chainring(s) fast.

Check out Sheldon Brown's article about chains. Towards the bottom of the page, he explains how to measure stretch and what it means.

http://www.sheldonbrown.com/chains.html

Basically, if the stretch

a) less than 1/16" per 12", you have nothing to worry about
b) 1/16": chain is streched, but chances are, the cassette is fine
c) 1/8", cassette is probably damamged.

I'm not sure how to interpret the LBS's assessment of "quite a bit", but with a tape measure, you can easily quantify the stretch get a feel for serverity.