Originally Posted by sydney
From Sheldon's own lips
Originally Posted by Sheldon Brown's Website
If the rivet is less than 1/16" past the mark, all is well.
If the rivet is 1/16" past the mark, you should replace the chain, but the sprockets are probably undamaged.
If the rivet is 1/8" past the mark, you have left it too long, and the sprockets (at least the favorite ones) will be too badly worn. If you replace a chain at the 1/8" point, without replacing the sprockets, it may run OK and not skip, but the worn sprockets will cause the new chain to wear much faster than it should, until it catches up with the wear state of the sprockets.
If the rivet is past the 1/8" mark, a new chain will almost certainly skip on the worn sprockets, especially the smaller ones.
The mark he refers to is a 12 inch mark.
Contrary to yours and several other's belief that all LBS employees are nosepicking idiots and pick pockets, in my shop we give customers all the options and possibilities we can. It is part of my own shop philosophy to maximize the life of all components whenever possible.
That said, if Old Gammy Leg came into my shop and the chain showed little or no stretch, I would tell him he's good to go. No new chain needed. But I would also check the cassette for wear just to be safe. AND I would do it at no charge. More often than not, by the time a customer decides they have a problem, the chain is over a link stretched and the cassette is toast.
Over the years I have discovered that hard and fast rules do not apply in the cycling world. Mileage on a chain is a good benchmark to check for wear, but not the final answer. As pointed out earlier, measuring is the only definitive way to tell. Personally I get well over 2000 miles on my road bike chains. My old Lotus may be approaching 4000. I am not sure. It's showing enough wear now though I will have to change the cassette if I decide to change out the chain. But it's in semi retirement now and gets little use. My newer roadbike, a custom build, is pushing 3000 miles on the original chain. It has hardly any stretch at this point. But I am a spinner, or maybe just a wimp. The bigger guys I ride with can toast a chain and cassette in 1000 to 1500 miles. On my mountain bike, it is a different story. Usually at least a new chain and cassette every season. I like to climb the tough technical climbs. They are brutal on chains.