This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Sprockets do not change pitch when they wear, only their tooth form
changes. The number of teeth and base circle remain unchanged by
normal sprocket wear.
A new chain often will not freely engage a worn rear sprocket under
load even though it has the same pitch as the chain. This occurs
because the previous (worn and elongated) chain formed pockets higher
on each tooth (a larger pitch diameter) than an in pitch chain
describes. This wear occurs because a worn chain rides high on the
teeth. A chain with correct pitch cannot enter the pockets when its
previous roller bears the previous tooth, because the pocket has an
overhang that prevents entry.
Without a strong chain tensioner or a non derailleur bicycle, the
chain has insufficient force on its slack run to engage a driven
sprocket. In contrast, engagement of a driving sprocket, the crank
sprocket, generally succeeds even with substantial tooth wear, because
the drive tension forces engagement.
However, worn teeth on a driving sprocket cause "chainsuck", the
failure of the chain to disengage. This occurs more easily with a
long arm derailleur, common to most MTB's, that is one reason this
occurs less with road racing bicycles, that experience a noisy
In contrast a worn chain will not run on a new driving sprocket. This
is less apparent because new chainwheels are not often used with an
old chain. In contrast to a driven (rear) sprocket the chain enters
the driving (front) sprocket under tension, where the previous chain
links pull it into engagement. However, because a used chain has a
longer pitch than the sprocket, previous rollers bear almost no load
and allow the incoming chain link to climb the ramp of the tooth, each
successive link riding higher than the previous until the chain jumps.
The pockets in a used sprocket are small but they change the pressure
angle of the teeth enough to cause skipping.
Jobst Brandt <email@example.com>