Rebuilt engines, frequently called overhauled engines, have been dismantled, inspected, cleaned, and their parts are replaced or repaired to update them to current manufacturing specifications. Rebuilding an engine, while labor intensive, costs less than replacing your entire vehicle or engine. Due to the unique process, some parts of rebuilt engines might be as old as the original, slightly used, or brand new, but many manufacturers will offer a warranty anyway. Choosing a rebuilt engine can extend the life of your car and might be a cost-effective option if you know you have serious engine trouble.

Technically, rebuilt engines are different from overhauled engines. With rebuilt, or remanufactured engines, a mechanic will either save or fix parts based on the amount of wear or damage. The original manufacturer of your engine, whether from an automobile or boat, is the only one authorized to remanufacture the engine. Much specialized machining is done to ensure that every part works well. However, overhauled engines contain only a few replaced parts, at full cost, and rarely involve machining old parts. This can be less effective in extending the life of the vehicle.

Some of the machining done in rebuilt engines includes reforming the piston-cylinder system. The cylinder channels are re-bored and the pistons are fitted to match the larger cylinders. All bearings are smoothed and lubricated. The crankshaft or camshaft might be reground, heads resurfaced, and connecting rods machined and precisely gauged. Other, less expensive parts, could be replaced, such as the timing belt, gaskets, and belts.

Since rebuilt engines have different parts with different operating histories, they are not guaranteed to last as long as a new engine. A part that was fine when the engine was evaluated during the remanufacturing process may malfunction after the engine has been re-installed. Yet, the advantage remains that it has been thoroughly inspected. If your vehicle stopped working for one reason, rebuilding the engine may lead to discovering, and preventing, other things that would have otherwise gone wrong.

Another advantage to rebuilt engines is that they will now meet current requirements regarding OEM, original equipment manufacturer, specifications. These might have changed since the engine was first produced, and their improvements might make your engine run smoother and more efficiently. If you have an engine with high mileage, especially over 100,000 miles (160,000 km), it might be worth considering rebuilding it.