Fairbanks-Morse 38 8-1/8: The Immortal Diesel

A WWII-era diesel engine that's too good to change

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Diesel engines are known to run forever. Anyone with a Ford 7.3L V8 diesel or a 5.9L Cummins can attest to this simple fact. Yet some engine designs are so robust that they remain largely unchanged for nearly a century.

The Fairbanks-Morse 38 8-1/8 is a 4 to 12-cylinder, two-stroke, opposed-piston diesel engine that was designed in the 1930s. It was used extensively during WWII aboard American submarines and numerous surface ships and continues serving today in both military and civilian applications.

Here’s why the Fairbanks-Morse 38 8-1/8 engine is both unusual and remarkable, and how this engine remained essentially the same over its 80-year lifetime.

Table of Contents


Fairbanks-Morse 38 8-1/8 History

Fairbanks-Morse wasn’t the first company to produce opposed-piston diesel engines. Soon after the advent of the internal combustion engine, manufacturers and engineers realized that both sides of the combustion chamber could be used to absorb power in a two-stroke cycle.


In the early 1930s, German aircraft company Junkers developed and produced a diesel-powered airplane using opposed-piston engines, but the project was abandoned due to its limited powerband and excessive weight.

Though the diesel-powered airplane concept didn’t initially take off, Fairbanks-Morse took a hint or two from the idea and turned the opposed-piston diesel into a remarkably efficient and powerful stationary engine. It continues in this role today, and will likely persist for decades more. New models are available, and numerous 1930s and 1940s-era FM 38 8-1/8 engines operated continually for decades and remain in service.

Design and Specs


The Fairbanks-Morse 38 8-1/8 is a 4 to 12-cylinder two-stroke opposed-piston inline diesel engine. It has two crankshafts and two pistons per cylinder. The engine was officially rated at around 3,400 hp, though an optional turbocharger increased its horsepower rating to 3,600 hp.

The Fairbanks-Morse opposed-piston diesel engine has a bore of 8-1/8 inches (206.4 mm) and a cylinder height of 38 inches (970 mm). It displaces about 12,444 cubic inches and operates at a BMEP of between 85 psi and 95.2 psi.

The base version of the 38 8-1/8 engine came without a supercharger, unlike many industrial diesel engines at the time. This is a two-stroke diesel engine, so it has no intake or exhaust valves. Instead, air is drawn in and exhaust gasses vent through fixed ports.

Tall and Narrow Configuration


The engine is tall and narrow. While this may seem unusual, it made sense at the time. The vertical triple-expansion steam engines that this diesel was designed to replace were also tall and narrow, and the unique design made it fit in many marine applications where width (not height) was at a much greater premium.

The bottom half of the engine should look somewhat familiar to any car enthusiast—it has an oil pan below the crankshaft and an upward-facing cylinder above it. But that’s where the similarities end. Above the bottom piston is an opposing piston connected to a top crankshaft.

The two crankshafts are connected on one side of the engine using gears and a vertical driveshaft. Air and fuel are injected through the sides of the cylinder, which has an extensive water cooling jacket to protect the vital parts from overheating.

All of the critical components of the engine can be accessed easily in tight spaces. The engine weighs about 30,000-40,000 lbs depending on the specific configuration, and it can make around 20,000 lb-ft of torque.

Benefits of the Opposed-Piston Design

Few industrial diesel engines use the opposed-piston design. So why did Fairbanks-Morse bet on it, and what are the advantages of using two pistons in each cylinder?

The opposed-piston design is highly flexible and doesn’t require a supercharger to achieve a good power-to-weight ratio. The two-stroke design of the 38 8-1/8 diesel and the lack of a supercharger gave it fewer moving parts and enhanced reliability.

But how does an additional crankshaft make the engine more simple? For one, there's no valvetrain at all. Each engine valve requires a pushrod, rocker arm, and perhaps a bearing, which is a notable increase in complexity. This engine doesn't need a cylinder head. That means there aren't any valve seats, valve seals, springs, or head gaskets to fail.

Opposed-piston designs also increase efficiency. With two pistons, the expanding combustion gasses have twice the surface area to exert usable force. In other words, power and heat aren't lost to a fixed cylinder head face. Also, opposed-piston engines achieve a higher stroke-to-bore ratio without excessive piston speed.

Fairbanks-Morse 38 8-1/8 Uses

The 38 8-1/8 diesel was used extensively in ships due to its reliability and slender configuration. It was easy to fit the engine in most ship designs of the era, and it worked well with seawater coolant. The engine also found extensive use in factories, locomotives, and power generation facilities.


Diagram of a WWII American Balao-class submarine. [Source]

The Fairbanks-Morse 38 was originally designed for industrial and marine use, and it continues to serve extensively in both capacities. Perhaps its most famous application was in United States Navy submarines during World War II.

The U.S. Navy equipped numerous Balao-class submarines with a set of four Fairbanks-Morse diesel engines. They were narrow enough to fit in a compact space, reliable, and provided easy access for crews to perform major overhauls at sea.

Aboard submarines, Fairbanks-Morse diesel engines drove generators that provided power for propulsion and charged batteries, as diesel could only be used on the surface.

Submarine tenders (vessels designed to provision submarines at sea) were also equipped with several 38 8-1/8 engines for propulsion, as they could be robbed for spare parts if a submarine needed them.

Additionally, the Navy continued to equip nuclear submarines with Fairbanks-Morse opposed-piston diesels as backup generators into the 1990s.

The U.S. Navy no longer uses diesel-electric submarines. However, several WWII subs equipped with Fairbanks-Morse engines have been preserved, including USS Pampanito, USS Ling, and USS Torsk.

Surface Ships

Fairbanks-Morse 38-series diesel engines were popular on ships of all sizes. Tugboats were often built with one or two of the engines, often with diesel-electric propulsion.

Along with submarine tenders, this Fairbanks-Morse diesel was used as a primary propulsion plant aboard Edsall-class destroyer escorts during WWII. It also served as a backup generator aboard numerous warships, including Essex-class aircraft carriers.  

Today, the engine can still be found aboard numerous American naval vessels. It’s also a relatively common choice for smaller auxiliary vessels such as tugboats, patrol boats, and a backup generator aboard cargo ships. One notable ex3ample is the lighthouse tender FIR-212, which was originally fitted with vertical triple expansion steam engines and later retrofitted with FM 38s.


Fairbanks-Morse 38 8-1/8 diesels had extensive use in small switching locomotives in the U.S. and Canada during the 1940s. They were also used by the Soviet Union in the TE3, which was the country’s first widely-produced diesel-powered locomotive.

During the decline of steam in the 1950s, the engines were widely adopted by locomotive manufacturers. However, they encountered problems at altitude that weren’t initially evident.

In ships and submarines, the engines had ample access to cool air at sea level. At altitude, water boils faster—and the closed-loop coolant system caused cylinder failures near the exhaust ports. Modifications were made, and the problem was resolved.

Fairbanks Morse Opposed-Piston Diesel Today

A version of this big Fairbanks-Morse diesel is still in production today in the United States. The modern 38 8-1/8 diesel engine is virtually unchanged. The modern engine, now designated Fairbanks Morse OP 38D 8-1/8, has been modified to use both diesel and natural gas. 

This dual-fuel model is used in factories, pump houses, hospitals, and numerous other applications. The company gives it a 40-year service life, though numerous WWII-era 38 8-1/8 engines still remain in regular or periodic service.

It's a case study for, "if it ain't broke, don't fix it."

Title Images Source: Fairbanks Morse

Diagrams: SFMNPA