Chapter 10 Engine Maintenance  and Operation Reciprocating Engine Overhaul Both maintenance and complete engine overhauls are  performed normally at specified intervals. This interval is  usually governed by the number of hours the powerplant  has been in operation. The actual overhaul period for a  specific engine is generally determined by the manufacturer’s  recommendations. Each engine manufacturer sets a total  time in service when the engine should be removed from  service and overhauled. Depending upon how the engine  is used in service, the overhaul time can be mandatory. The  overhaul time is listed in hours and is referred to as time  before overhaul (TBO). For example, if an engine had a life  of 2,000 hours and had operated 500 hours, it would have a  TBO of 1,500 hours. Tests and experience have shown that  operation beyond this period of time could result in certain  parts being worn beyond their safe limits. For an overhauled  engine to be as airworthy as a new one, worn parts, as well  as damaged parts, must be detected and replaced during  overhaul. The only way to detect all unairworthy parts is to  perform a thorough and complete overhaul process while the  engine is disassembled. The major purpose of overhaul is to  inspect, repair, and replace worn engine parts.