I had a used 5.9 put in place of my original engine in my 98 Ram a few weeks ago. On the replacement engine the idler and water pump pullies both had rust. Should I have assumed that the mechanic was going to replace them with the original ones or just leave the bad ones on to replace later? I am done with this mechanic because of other dealings or I would ask him about it.

Are you talking about surface rust on shiny contact area of pullies from donor engine sitting for a bit? If so that is usually left alone and it may accellerate belt wear a little.

Don't assume anything.

The pullies are pitted and have quarter size rust that is raised up enough to make the belt slip to one side and chirp loudly. It is from sitting. I don't normally assume but hearing nothing but good things and being a friend of my father's, I did. Never again though. Going outside now to change the water pump one.