it has been my experience that the whole pulley has to be replaced. i have a 1989 C1500 4.3 and have replaced 4 of them since i bought the truck in '92. it's a snap on my truck, take off the belt, and one bolt holds it on.
if you are talking about the belt tensioner pulley, it is the same thing, just remember that there is a little nipple on the back of the pulley that needs to be lined up correctly in a hole on the mounting plate, if you don't get it right, it will easily break off. i found that one out the hard way.
for my truck, the idler was about $15 US and the tensioner was about $30 US. usually as a precaution i go ahead and change the belt.
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