Several inline-six engines have earned strong reputations for durability, but there is no evidence here to rank them by measured failure rates. SlashGear’s September 16, 2023 roundup names ten notable examples; their strengths range from simple work-engine design to performance engineering, and each comes with different maintenance demands. Treat this as a guide to reputations and reported caveats, not a guarantee about any engine or vehicle.
What “reliable” means in this list
The engines below appeared in SlashGear’s 2023 roundup. That article is a secondary editorial ranking, not a comparative reliability study: it supplies no common failure-rate data, repair-cost figures, or test conditions for the ten engines. An engine’s real-world condition also depends on maintenance, age, operating conditions, modifications, and the health of the vehicle around it. A reputation for surviving tuning or hard work is not the same thing as a measured service-life estimate.
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The descriptions and historical figures below are attributed to SlashGear’s September 16, 2023 article; they should be read as reported context rather than independently verified specifications.
The 10 inline-six engines
1. BMW M30
SlashGear says BMW introduced the M30 in 1968 and built it through 1995, a 27-year production run, with displacements growing from 2.5 to 3.5 liters. The engine is presented as a durable design with aftermarket support. Ownership still calls for appropriate upkeep, including valve adjustments; an old engine’s reputation cannot substitute for checking its maintenance history and present condition.
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2. Mercedes-Benz M103 and M104
The M103 is associated with W124-era 300E models. SlashGear describes the M104 as adding a second camshaft and another set of valves. The article’s broad longevity claims are conditional on well-maintained cars, and it notes that regular maintenance matters alongside the greater complexity of the vehicle. These are not maintenance-free engines simply because they have a favorable reputation.
3. AMC/Jeep 4.0-liter
SlashGear dates the revised 4.0-liter’s arrival in Jeep Cherokee, Comanche, and Wagoneer applications to 1987, and says Chrysler continued producing it through 2006. Its appeal is a relatively simple, repairable design with useful low-end torque. The article also flags oil leaks, coolant leaks, and noises as possible issues in high-mileage examples. A durable engine does not make an aging Jeep’s cooling system, transmission, suspension, or body sound; assess the whole vehicle.
Rank #2
4. Hudson Twin H-Power
The Hudson Hornet’s Twin H-Power setup paired a 5.0-liter flathead six with two carburetors. SlashGear connects the Hornet with NASCAR success, reporting 79 wins from 1951 to 1955, and emphasizes simplicity as part of the engine’s appeal. That racing history is interesting context, not modern fleet-reliability evidence, and a historic car’s condition depends heavily on its preservation and maintenance.
5. Chrysler Slant Six
Chrysler’s Slant Six used an overhead-valve design tilted 30 degrees for packaging, according to SlashGear. Its straightforward, economical character helped build its reputation. The article distinguishes that basic design from later versions with electronically controlled carburetors, which could become troublesome as emissions requirements changed. For a specific car, identify its equipment and emissions configuration rather than assuming all Slant Six applications are alike.
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Rank #3
6. Ford Barra
Ford Australia introduced the 4.0-liter DOHC Barra in 2002, in naturally aspirated and turbocharged forms. SlashGear describes stock, un-abused engines as robust and mentions oil leaks as a possible issue. The Barra’s tuning reputation should not be confused with proof of a particular stock engine’s expected lifespan: modifications, calibration, use, and upkeep all affect the example being considered.
7. Ford 300
The Ford 300 is presented as a low-speed-torque engine for trucks and industrial applications. SlashGear highlights its iron block and head and relatively simple repairability, while noting possible oil and exhaust leaks. Its work-oriented design and service context differ from a high-revving performance engine, so the relevant questions include how it was used and whether repairs and routine service were kept up.
Rank #4
- Used Book in Good Condition
8. Nissan RB26DETT
Best known here as the twin-turbo inline-six associated with the Skyline GT-R, the RB26DETT is described by SlashGear as overbuilt. The same article warns of oil-pump problems in some early examples and makes its positive reliability assessment conditional on maintenance and use. Motorsport success and tuning fame do not establish ordinary-use dependability; a prospective buyer should pay particular attention to the specific engine’s history and condition.
9. Toyota 2JZ-GE
The 2JZ-GE is the naturally aspirated engine named in this entry. SlashGear discusses the 2JZ in the Supra context and points to its engineering reputation and tuning headroom, while also discussing power in connection with turbocharged Supra variants. Those turbo-related claims should not be transferred wholesale to the GE: the GE and turbo GTE are distinct versions, and tuning potential is not a reliability statistic.
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SlashGear says the 5.9-liter Cummins arrived in Dodge pickups in 1989 and describes later development into a 6.7-liter version. This is a diesel truck engine, not a direct like-for-like counterpart to the passenger-car and performance engines elsewhere on the list. Its heavy-duty application and maintenance regime shape what durability means; compare it with other truck diesels in similar service, not simply by reputation against a car engine.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to use the reputations when evaluating one
These examples illustrate different routes to a durable reputation: simple, repairable work engines such as the Ford 300 and Jeep/AMC 4.0; performance-oriented engines such as the Barra, RB26DETT, and 2JZ-GE; and designs whose age, complexity, or specialized use make application-specific care essential. The source does not establish which has the lowest failure rate or the cheapest ownership costs.
Quick Recap
- Identify the exact engine version and vehicle application; names can cover different configurations, as with the Barra and 2JZ family.
- Ask for service records and evidence that known maintenance needs were addressed, including valve adjustment on an M30 and oil-system concerns on early RB26DETT examples.
- Inspect for the reported weak points relevant to that engine, such as leaks on the Jeep/AMC 4.0, Ford 300, or Barra.
- Account for modifications and intended use. Tuning strength, race history, or a work-truck reputation does not guarantee the condition of a particular engine.
- Judge the entire vehicle, especially when an otherwise durable engine sits in an old or heavily used car or truck.
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