In the winter of 1933, in a design office of a company that had spent the previous century building scales, windmills, and water pumps, two engineers named Heinrich Schneider and Percy Brooks filed a patent for something that looked utterly bizarre on paper. An engine without cylinder heads, an engine with two pistons facing each other inside the same cylinder, moving toward each other like two fists about to collide with an explosion trapped between them. Most engineers who saw the drawing assumed it was just another novelty, a strange contraption destined for museum shelves alongside other ambitious failures of the machine age. No one in that room could have imagined that the fruit of this drawing would, within a decade, determine the outcome of the Pacific War, hidden inside steel hulls beneath an ocean that had not yet become a battlefield.

Half a world away, in those same years, other engineers were racing toward entirely different breakthroughs beneath the waves. History would ultimately judge those German efforts as technically brilliant but practically fragile. Meanwhile, the opposed-piston diesel engine, lacking in glamour, proved itself as a far more decisive machine. This is the story of the Fairbanks-Morse 38D8 1/8—an engine barely mentioned outside naval history, yet it powered the submarines responsible for strangling an empire. An engine still manufactured today, nearly nine decades after its first patent, defying all rules of military technology obsolescence.

To understand why the U.S. Navy needed such an engine, one must first understand the company that built it. Fairbanks-Morse was among the least likely candidates to become a cornerstone of submarine warfare. Founded in 1823, the company spent generations manufacturing industrial scales—the kind used to weigh grain, livestock, and freight. Over the decades, it expanded into windmills, coffee grinders, water pumps, and agricultural equipment. Based in Beloit, Wisconsin, Fairbanks gradually earned a reputation as a solid, unglamorous manufacturer of the American Midwest, thanks to its Model 38 engine.

The engine began with a relatively modest design of six cylinders, 5-inch bore and 6-inch stroke, originally equipped with a chain-driven overhead crankshaft connecting the two crankshaft groups required by the opposed-piston architecture. By 1938, the company introduced a welded steel frame, replacing the boxy cast-iron block of earlier versions, and developed a larger 8-inch bore version that immediately caught the U.S. Navy’s attention.

In December 1934, the Navy ordered eight early 38A engines. In a moment of remarkable candor, the Navy informed Fairbanks-Morse that the engine, in its current design, was four inches too tall to be properly maintainable within the cramped space of a submarine engine room. Most companies receiving such criticism from their only realistic customer might have abandoned naval contracts entirely. But Fairbanks-Morse went back to the drawing board, redesigning the engine block to remove exactly four inches of height above the intake manifolds and shortening the upper connecting rods to match—thus creating the engine that would later be known as the 38D 1/8.

The letter D indicated a major design revision, while the numbers 8 and 1/8 referred to the cylinder bore in inches, paired with a 10-inch stroke for each of the two opposed pistons. The resulting mechanical arrangement was utterly different from almost anything else operating in the engine rooms of the world’s navies.

There were two complete crankshafts, an upper and a lower, connected by gears and synchronized through a vertical drive system damped by a spring pack to absorb the torsional shock of combustion. Each cylinder liner contained two pistons: an upper piston connected to the upper crankshaft and a lower piston connected to the lower crankshaft. The pistons converged toward the center of the cylinder during the compression stroke and retreated outward during the power stroke.

Intake ports were machined directly into the cylinder liner near the top, uncovered by the upper piston at the end of its stroke, allowing air from a Rootes-type blower to flow into the cylinder and scavenge the burned exhaust gases. Exhaust ports were located near the bottom of the liner, uncovered by the lower piston at the corresponding moment of its stroke, discharging burned gases toward the collection manifold.

Because it was a two-stroke design, every downward stroke of the pistons was a power stroke, with no wasted cycles as in four-stroke engines that require intake and exhaust strokes. There were no cylinder heads, no head gaskets, and no valve trains requiring maintenance. None of the components that typically failed first and most frequently in conventional diesel engines under the harsh operating conditions and severe vibrations of naval service.

In the eight-cylinder inline configuration, the engine displaced over 1,000 cubic inches per cylinder and could be manufactured, depending on application, in versions ranging from four to twelve cylinders, raising output from a few hundred horsepower to over 2,000.

The timing of this engineering achievement was critical, as the U.S. Navy was about to enter a war that would demand more from its submarine force than any navy in history. After the attack on Pearl Harbor in December 1941, the Pacific Fleet’s surface combat force suffered devastating losses in a single morning. Submarine commanders turned to the only weapon capable of immediate retaliation from long range.

Naval strategic doctrine shifted toward what historians now call the strangulation strategy, sending long-range fleet submarines into deep waters controlled by Japan to sever the maritime supply lines the island empire desperately needed for fuel, raw materials, and reinforcements. This was an entirely different kind of submarine warfare than the fleet submarines had been designed for before the war. It required engines capable of operating efficiently on patrols lasting weeks across thousands of miles of open ocean. Engines that could be repaired by sailors working in cramped, dark compartments far from any shipyard. Engines capable of absorbing the violent shock of nearby depth charge explosions without failing.

The four-stroke diesel engines in many early fleet submarines, including some troublesome designs from other manufacturers, proved frustratingly unreliable under these conditions, with broken crankshafts, damaged fuel injectors, and submarines forced to return to port early at the very moment the Navy desperately needed every available boat on station.

Submariners needed something more robust, more durable, and far easier to maintain than what they had. Fairbanks-Morse, almost by accident, and thanks to German aviation engineering from a decade earlier, already had the solution in its catalog.

What followed was one of the most remarkable industrial mobilizations of the entire war. And it did not happen in a gleaming coastal shipyard, but in the inland industrial town of Beloit, Wisconsin, hundreds of miles from any ocean where those engines might ever operate.

Fairbanks-Morse ranked sixtieth among American companies by dollar value of wartime military production contracts during World War II. A stunning achievement for a company that had been making scales and coffee grinders a generation earlier. The Beloit plant raised output of the 38D 1/8 at a pace that would have seemed impossible before the war.

Like most American war industries, the plant relied heavily on an unprecedented factory workforce. A labor force, predominantly women, assembled, manufactured, and tested these highly complex dual-crankshaft engines, learning the intricacies of opposed-piston timing and turbocharger calibration within months.

By the end of production, roughly 2,300 of these engines had left the Beloit plant and been installed in American submarine hulls, surface escort vessels, and other warships. An output that exceeded everything the company had produced before the war.

These engines went to war in two of the most famous submarine classes in U.S. Navy history: the Gato class and the Balao class. These submarines, roughly 311 feet long, typically carried four Fairbanks-Morse engines, designed to drive electrical generators in a diesel-electric propulsion system, since direct mechanical drive was impractical for a vessel that needed to switch instantly between surface operation and underwater electric power.

Submarines like USS Pampanito, which later sank Japanese transports and rescued Allied prisoners of war from the open ocean after a friendly-fire torpedo attack on an unknown Japanese ship, relied on this engine to cross the vast distances of the central Pacific. USS Torpedo, which later fired some of the last torpedoes of the war, depended on the same basic mechanical architecture inspired by that strange German aviation patent issued a decade earlier.

The difference in reliability these engines provided was not a minor improvement—it was the difference between a submarine capable of maintaining an intense wartime patrol schedule in the Pacific and one limping back to base for repairs after a few weeks at sea.

Submarine commanders quickly discovered that depth charge attacks, which could shake a submarine’s hull and machinery with tremendous force, often left Fairbanks-Morse engines running while damaging other, more sensitive systems aboard. The absence of an exposed cylinder head—the very feature that had made the design look strange to engineers a decade earlier—turned out to be precisely the feature that allowed these submarines to survive damage that would have disabled a less robust power plant.

The strategic impact of this reliability extended in ways that reshaped the entire Pacific War. American submarines, operating with engines that were relatively easy to repair by trained technicians working in cramped engine rooms, began sinking Japanese merchant shipping at a rate the Imperial Japanese Navy could not adequately counter.

Japan, an island nation utterly dependent on seaborne imports of oil, rubber, iron ore, and food to sustain its military and civilian population, watched its merchant fleet shrink under a campaign that historians now regard as one of the most decisive uses of submarine warfare ever conducted, and arguably more strategically significant per ship than the famous German U-boat campaign in the Atlantic.

Despite receiving far less attention in postwar popular memory, Japanese convoy escort doctrine, built on the assumption of a much smaller submarine threat, never kept pace with the scale of the onslaught. Crude oil ran short at the refineries; aluminum ran short at the aircraft factories. By the war’s final year, Japanese warships, including some of their largest, suffered acute fuel shortages, curtailing exercises and forcing operational planning based on how much fuel remained rather than available firepower.

None of this was due to a single engine, of course. It was thanks to sophisticated codebreaking intelligence, which often directed American submarines straight onto Japanese convoy routes; to torpedo designs that improved dramatically after their early-war defects were corrected; and to the courage of submarine crews who operated in genuinely terrifying conditions far from any chance of rescue.

But behind every one of those patrols was a mechanical foundation that worked when it mattered. Patrol after patrol, unlike earlier American submarine engines.

It is worth noting here that Fairbanks-Morse was not alone in the field of submarine propulsion. General Motors, through its Cleveland Diesel Engine Division, produced conventional four-stroke engines that powered a large portion of the American submarine fleet alongside Fairbanks-Morse boats. The rivalry between the two engine philosophies raged across the Pacific throughout the war.

The Cleveland Diesel 16-278 became one of the most widely used powerplants of the war, eventually producing around 1,800 horsepower, a figure that made it roughly comparable to later generations of diesel-electric engines. Cleveland Diesels were lighter and easier to manufacture using conventional tooling, and crews who served aboard Cleveland-powered submarines often developed a particular loyalty to those machines, sometimes teasing opposed-piston boats with good-natured criticism as overly complex.

But the opposed-piston design, thanks to its freedom from cylinder-head failures, gave the Fairbanks-Morse engine a special reputation for enduring the kind of violence that depth charge attacks and emergency dives inflicted on the hull. Submariners, accustomed to forming close personal relationships with the machines that saved their lives, often spoke of the 38D8 1/8 with considerable affection. An unusual sentiment toward a cast and welded steel mass weighing several tons.

Even the Navy’s Engineering Experiment Station in Annapolis, Maryland, spent years after the war conducting comparative tests between the two designs, experimenting with early turbocharging concepts partly inspired by the small experimental Detroit Diesel 171 engine, pursuing combustion pressures and horsepower figures that would have seemed impossible to the engineers of either company during the war itself.

The war did not end the engine’s career, though it nearly did. In an episode that reveals how close this story came to a quiet ending, in the years immediately following the war, Fairbanks-Morse sought to market its opposed-piston design in a completely new market: the American railroad industry, hoping to compete with the diesel-electric locomotives being produced by companies like the Electro-Motive Division and the American Locomotive Company, known to railroaders as Alco.

Initial results looked promising. Fairbanks-Morse locomotives built around the 38D8 1/8 performed well in railroad yard service, and the company’s powerful, oddly styled locomotives built in 1945, which ran the engine at a brake mean effective pressure far higher than naval specifications, attracted genuine interest from major railroads. But mainline service revealed weaknesses the Navy had never encountered.

Submarines drew cooling water from the open ocean, with near-limitless access to cool surface air. Locomotives crossing the high desert of the Union Pacific ran closed-loop cooling systems under brutal thermal stress, often trailing a hot exhaust plume. Pistons that performed flawlessly inside a submarine hull began cracking near the exhaust ports. It took Fairbanks-Morse nearly eight years of metallurgical development to produce a piston durable enough for sustained mainline service—by which time the damage to the locomotive program’s reputation was done.

By 1963, Fairbanks-Morse had withdrawn entirely from the locomotive market. Yet the Navy never stopped relying on the engine. When the experimental “pancake” engine for the Tang class proved mechanically impractical, the Navy quietly replaced the failed units with proven Fairbanks-Morse powerplants from World War II.

Then came nuclear power. Logic suggested a 1930s diesel had no place aboard a reactor-driven submarine—yet the Navy installed a slightly modified variant, the 38 ND8 1/8, as emergency backup. If the reactor scrammed, the old opposed-piston engine could still turn the boat’s electrical systems and even provide limited propulsion.

Today, Fairbanks-Morse Defense still builds it in Beloit, Wisconsin, now capable of running on diesel or natural gas. The aircraft of World War II became museum pieces; the submarines that carried this engine are gone. The engine simply kept working.

What do you think—does any machine today stand a chance of serving for ninety years?