How reliable was the Slant Six engine?

The Slant Six, Chrysler’s 225-cubic-inch inline-six, earned a solid reputation for durability and straightforward maintenance. In practice, it was renowned for long life and dependable performance, with many engines logging hundreds of thousands of miles when properly cared for.

This article examines the design choices that supported reliability, the common issues that could arise over decades of use, and how owners and enthusiasts have kept these engines running long after their debut in the 1960s.

Design features that underpinned reliability

To understand its reputation, it helps to look at the design decisions that made the Slant Six a durable workhorse.

Key design elements

• Robust bottom end: a sturdy iron block and reliable crankshaft setup provided a durable foundation able to tolerate long hours and moderate rpm without dramatic wear.


• Simple OHV pushrod valvetrain: a straightforward cam-in-block design with relatively simple maintenance contributed to predictable wear patterns and easier servicing in garages of all sizes.


• Low-to-mid range torque: strong torque at common driving speeds reduced peak stress on components, helping the engine feel resilient in everyday use.


• Ease of maintenance and parts availability: broad aftermarket support and a large supply of affordable parts made routine repairs inexpensive and straightforward.


• Carbureted fuel delivery and durable ignition: the engine’s uncomplicated fuel and ignition systems were designed for reliability and ease of troubleshooting, which paid off in fleet and rural use.

Taken together, these design choices created a powerplant that could endure rough service and high mileage with comparatively little complexity.

Common reliability concerns and weaknesses

Even stalwart engines have areas that demand attention as they age. The Slant Six is no exception, and understanding these points helps explain variations in reliability across different vehicles and maintenance histories.

Typical failure modes

• Overheating risks: a neglected cooling system or cooling-system component failures could push the engine toward head gasket issues or other heat-related wear.


• Head gasket and head wear: repeated or severe overheating could lead to gasket leaks and, in extreme cases, head damage that compromised seal integrity and performance.


• Fuel and ignition wear: aging carburetors, distributors, and ignition components could cause rough running, reduced efficiency, and lower reliability if not refreshed.


• Valve-train wear: as miles accumulate, valve guides and seats may require inspection or refurbishment during a rebuild to preserve performance.


• Emission-era updates and compatibility: some later versions saw changes for smog controls, which could affect tune-ups and maintenance practices for drivers used to earlier configurations.

With consistent maintenance—especially keeping the cooling system healthy and replacing worn ignition and fuel-system parts—the Slant Six remained dependable well into its later years.

Real-world usage and legacy

Across its production life, the Slant Six powered a wide range of Mopar cars, vans, and light trucks. In everyday service, fleets and private owners alike praised its ease of maintenance, generous low-end torque, and ability to run reliably with relatively modest maintenance budgets. Enthusiasts today continue to restore and preserve these engines, thanks to a robust aftermarket and plenty of affordable used parts.

Its enduring legacy is reflected in the way modern restorers approach the Slant Six: prioritize cooling-system integrity, ensure solid lubrication, and address wear in the valve-train and fuel system. When those basics are tended to, the engine’s reputation for longevity remains well earned.

Summary

The Slant Six earned its standing as a reliable workhorse thanks to a durable bottom end, a simple and maintainable valvetrain, and a layout that tolerated miles and rough service. While overheating, gasket issues, and wear in aging fuel and ignition components could erode reliability if neglected, routine maintenance largely prevented these problems. For decades, it served as a foundational engine in Mopar’s lineup and remains a beloved model among collectors and drivers who prize long-term durability.

What is the most reliable Mopar engine?

The most reliable Mopar engines are often considered to be the 225 Slant 6 for its near-indestructibility and the 5.7L and 6.1L HEMI V8s for their power and proven durability, with the 5.2L and 5.9L Magnum V8s also highly reliable. For historical reliability and simplicity, the 225 Slant 6 is a top choice, while modern and performance-oriented reliability often points to the HEMI and Magnum series.
 
Historically reliable

• 225 Slant 6: Often cited as one of the most reliable engines ever produced, it is known for its incredible durability and simple, robust design, even if it lacks high horsepower. 
• 5.2L and 5.9L Magnum V8s: These engines are considered very reliable, with many owners achieving over 350,000 miles with proper maintenance. 
• 318 V8: A smaller and less powerful version of the Magnum, the 318 is also known for its reliability and simplicity. 

Modern and performance-oriented reliable 

• 5.7L HEMI: Widely regarded as a reliable engine, it is used in many modern Ram trucks, where its towing capability is a testament to its strength. 
• 6.1L HEMI: Generally considered reliable, with its lack of Variable Valve Timing and Multi-Displacement System (MDS) making its design simpler and more durable than some later HEMI versions. 
• 3.6L Pentastar V6: This modern engine powers many Jeeps, Dodges, and Rams, but its reliability can vary depending on the model year and specific issues that may arise. 

Engines for a different purpose

• 426 Hemi: While legendary for its power, it was designed as a race engine, and durability was not the primary concern, especially in the early competition models.
• 440 RB: This engine is powerful and simple to work on, but its high-performance variants were often run very hard, impacting long-term durability in comparison to less powerful engines. 

Is the slant 6 engine reliable?

It Was Exceedingly Reliable
One of the hallmarks of the Slant-Six's reliability was its use of something called a deep-skirt design, which means its crankshaft is mounted above the oil pan rails, leading to better structural durability and integrity.

What is the advantage of the Slant 6 engine specs?

Efficient cooling and lubrication systems, a favorable ratio of connecting rod length to stroke, and a forged steel crankshaft (on engines made through mid-1976) all contributed to the engine's strength and durability.

What was the last car with a slant 6 engine?

The last passenger cars in the US with the 225 Slant-Six were the 1983 M-body cars (Chrysler Fifth Avenue, Dodge Diplomat, Plymouth Gran Fury). The last vehicles in the US were the 1987 Ram pickups and vans, after which it was replaced by the new 3.9L V6 (basically a 318/5.2L with two cylinders lopped off).