If it almost feels smoth (Not much of a pit) when you run your finger
across the surface and pass over top of where one of the round "rivet"
things are, then replace it...
However... once you've pulled the engine and can get to the thing, 
you might as well replace it if it's even close.  Maximumize the
interval between engine pulls.

Cheers

<sstones@io.org>
SStones             Toronto,  Ontario.

You can lead a horticulture, but you can't make her think.