Frank Krygowski wrote:

Chain wear happens inside the chain's pivots.
The wear is on the pins, the rollers and any
surface between them.

The wear happens when the pivot is under
tension and the two links connected by that
pivot are either bending or straightening.

OK...? Do you have a picture that illustrates
this or at least identifies the parts refered
to? I have this

http://user.it.uu.se/~embe8573/bike/...erminology.jpg

but "pivot" isn't mentioned...

And how long should you be able to go before
the slack exposes you a fraud mechanic
and/or the owner of crooked gear?

For derailleur bikes, many people recommend
replacing a chain when it's worn (or
"stretched") so it's one half percent longer
than when new. (This is most easily measured
with a ruler marked in inches. Do you have
one of those?)

Eheh... what's next, you going to ask Picasso
if he has a brush? Of course I have folding
rule, ruler, measuring tape, and calipers in
inches as well as cm.

But you have to remove the chain to do this,
right? So it is not so fast after all.
Even less so with a chain guard which is very
common on these bikes.

The method I use is: see if the chain is slack.
If it is, move the wheel back. If it can't be
done, get a new chain. Some people remove links
but chains are so cheap anyway.

--
underground experts united
http://user.it.uu.se/~embe8573