"rebuilt" is a subjective term. But typically during a rebuild ONLY parts that are worn beyond use, damaged or have some defect are replaced. For instance it use to be the difference between a 1yr warranty alternator and a lifetime (Not Brand new) alternator was the 1yr warranty only had parts that were bad replaced and MAYBE new bearings and/or brushes. A lifetime warranty alternator would have brand new brushes, bearings and voltage regulator. During an engine rebuild the crankshaft is reconditioned or replaced if it is beyond specs for reconditioning. The pistons are usually replaced. In a transmission rarely are the hard parts replaced unless of course they are damaged, worn or have a defect. But the tranny is still rebuilt.....using many of the original parts that were installed from the factory.