Some engines earn a reputation for running almost indefinitely when owners stick to basic maintenance, and mechanics often point to a familiar group of workhorse designs. From simple mechanical diesels to overbuilt Japanese inline engines, these powerplants show how conservative engineering and regular service can translate into extraordinary longevity. The following ten engines, drawn from reporting on the most reliable designs in the World, illustrate what “runs forever” looks like in practice.Toyota 22R-EThe Toyota 22R-E is described as a naturally aspirated 2.4-liter four-cylinder that has earned a reputation for being “nearly indestructible” on the durability front. Reporting on the Toyota 22R-E notes that this engine can outlast the vehicle around it when owners keep up with oil changes and cooling system checks. Its simple fuel injection, non-interference design and conservative output keep internal stresses low, which is exactly what long-term mechanics look for when they talk about engines that “run forever with basic maintenance.” For drivers, the implication is clear. A well maintained 22R-E in an older Toyota pickup or 4Runner can still deliver reliable daily use, even with high mileage, because the engine’s design tolerates wear and imperfect conditions better than many modern, highly stressed units.The Mercedes 3.0L OM617The Mercedes 3.0L OM617 inline five-cylinder diesel engine is described as “considered by many to be the most reliable engine in the world.” Coverage of the Mercedes OM617 highlights examples still running with over 500,000 miles, which directly supports mechanics who say these diesels can run almost indefinitely if owners change oil and fuel filters on schedule. The mechanical injection system and lack of complex electronics reduce failure points and make basic servicing straightforward. For owners of classic Mercedes sedans and coupes, that durability translates into lower long-term costs and confidence in high-mileage purchases. The OM617 shows how a relatively low specific output diesel, paired with regular fluid changes, can deliver decades of service that few modern engines can match.Ford 300 Straight-6The Ford 300, a 4.9-liter inline-six, is frequently cited by mechanics as a textbook example of an engine that will not quit. Reporting on the Ford 300 describes it as a big-bore, long-stroke design built for heavy-duty work, used not just in trucks but also in forklifts, generators, and farm machinery. The same source notes that its low-end torque and simple construction made it ideal for continuous, hard use. Enthusiasts and fleet operators value this engine because it responds well to basic maintenance, such as regular oil and coolant changes, and tends to tolerate abuse that would sideline more delicate designs. The repeated use of the number 300 in discussions of durable power plants reflects how strongly this engine is associated with longevity in the Ford lineup.Chrysler 225 Slant SixThe Chrysler 225 Slant Six, often referred to simply as The Chrysler Slant Six, appears repeatedly in discussions of engines that “last forever.” Coverage of the Chrysler 225 Slant Six notes that it was a fantastic, reliable engine and that the only real negative was that many owners did not realize it had solid lifters that required periodic adjustment. When that basic maintenance was performed, mechanics saw these engines accumulate very high mileage with minimal internal work. Another report on long-lived engines highlights the Chrysler Slant Six as part of a group that has “stood the test of time,” with references to a 1963 Plymouth Valiant Signet 200 Convertible Chrysler Slant that still runs. For buyers of vintage Mopar cars and trucks, this history suggests that a properly serviced 225 can remain dependable transportation long after bodywork and interiors have aged.Cummins 6BT 5.9LThe Cummins 6BT 5.9L, often described in reporting as a benchmark for mechanical diesel durability, is a favorite among mechanics who prioritize engines that can run forever with simple service. A guide on An Unkillable Example calls The Mechanical 6BT Cummins an “unkillable example” of a reliable diesel, pointing to its mechanical injection system and fixed geometry turbocharger as key reasons for its longevity. Another technical overview of the Cummins 6B family notes that the 6B is a naturally aspirated 5.9L inline-six diesel with no turbocharger, which simplifies maintenance and reduces stress. For truck owners and equipment operators, these engines show how basic oil, filter, and coolant service can keep a heavy-duty diesel working for hundreds of thousands of miles.Toyota 2JZ-GE / 2JZ-GTEThe Toyota 2JZ-GE and 2JZ-GTE, both 3.0L inline-six engines, are widely regarded as overbuilt for their original applications. A technical summary on Legendary Durability describes the Toyota 2JZ-GE/GTE as a 3.0L Inline-6 that can reach “300k+ miles easily,” and notes that it was Overbuilt for turbos, with stock internals that handle massive power increases. That same overengineering means that in stock form, with regular oil changes and cooling system care, the 2JZ can run for decades. For owners of Toyota Supra models and Lexus sedans that use the 2JZ family, this durability translates into strong resale values and a thriving tuning community. Mechanics often point to these engines as examples of how conservative factory tuning and strong bottom-end components create an almost unbreakable platform when basic maintenance is respected.Toyota 2.5L 4-Cylinder (2AR-FE)The Toyota 2.5L 4-Cylinder 2AR-FE is highlighted in a survey of the Top 10 Most Reliable Engines in the World as a standout for modern reliability. Reporting on Top Most Reliable Engines notes that this Cylinder is used in mainstream models such as the Toyota Camry, RAV4 and Lexus ES, and that its design prioritizes long life and low maintenance costs. The combination of chain-driven cams and efficient cooling allows it to handle high mileage with only routine service. For families and commuters, that reputation means the 2AR-FE can be trusted for long-distance driving without fear of major engine failure, provided oil changes and basic inspections are kept current. Mechanics often recommend these cars to buyers who want a modern, efficient engine that still behaves like the old “runs forever” designs.Toyota 1UZ-FE UZ-family trucks and SUVsThe broader Toyota UZ-family, including 1UZ-FE derivatives used in trucks and SUVs, appears in reporting on long-lived engines that have stood the test of time. A feature on Toyota truck V8s highlights the Toyota 1/2/3UZ-FE as used in the 2000 to 2002 Toyota Tundra 1st Gen (XK30) Engine Bay and in the 2003 to 2009 Lexus GX 470. These engines carry over the durability traits of the original 1UZ-FE, including strong internals and conservative tuning. For owners of early Toyota Tundra and Lexus GX models, that shared heritage means the same basic maintenance that keeps a Lexus LS400 running can also keep a work truck or family SUV on the road. Mechanics often see these UZ-family engines reach high mileage with minimal issues, reinforcing Toyota’s reputation among the top reliable engine manufacturers alongside Honda, General Motors, and More.