B10 Engine Life
The expected engine life, measured in miles of operation, before 10 percent of all engines in operation will require major repair(s), overhaul, or replacement.
Example: The 7.3L Power Stroke has a B10 life of 200,000 miles. Therefore, 10 percent of all engines in operation can be expected to require an overhaul at/near 200,000 miles.
B50 Engine Life
The expected engine life, measured in miles of operation, before 50 percent of all engines in operation will require major repair(s), overhaul, or replacement.
Example: The 5.9L Cummins has a B50 life of 350,000 miles. Therefore, half of the engines in operation can be expected to require an overhaul at/near 350,000 miles.
An "overhaul" or "major repair" is considered a repair that requires removal of the cylinder heads or dropping of the oil pan. Failure/replacement of components that are accessible without removing the heads or dropping the oil pan do not attribute to B10/B50 life expectancy measurements.
B10 & B50 life are statistical measurements of engine life. As such, they don't represent a definite time when your engine will require a repair or overhaul. Proper maintenance and care will ensure that you get the most from your engine. Following the OEM maintenance recommendations will help ensure that your engine meets of exceeds the manufacturers life expectancy.