An engine swap is the process of removing a car's engine and replacing it with another. This is done either because of failure, or to install a different engine, usually one that is more powerful or more modern and maintainable. An engine swap can either be to another engine intended to work in the car by the manufacturer, or one totally different. The former is much simpler than the latter. Fitting an engine into a car that was never intended to accept it may require much work – modifying the car to fit the engine, modifying the engine to fit the car, and building custom engine mounts and transmission bellhousing adaptors to interface them along with a custom built driveshaft.