The engine break-in period is a period of time after an engine is built or rebuilt during which the various parts of the engine wear in and work together properly. The engine is subjected to a variety of conditions and loads to seat the rings, hone the cylinders, and wear in the bearings and other moving parts. This process helps to ensure that the engine runs smoothly and efficiently once it is fully broken in. It is important to follow the manufacturer's recommended break-in procedures to ensure that the engine is properly broken in and protected.