A car engine overhaul or engine rebuild is usually done when there are serious problems with a car and its engine. Occasionally, it is also performed on worn engines with higher mileage to restore them back to their factory condition. During an overhaul, your mechanic disassembles the engine and then evaluates the condition of the internal components to identify the source of the problems and fix them. Internal parts such as the piston rings, bearings, gaskets, and camshaft may be replaced during an overhaul, though it depends on their condition and what the source of the problem is.