America Had No Tank Engines in 1940 – So GM Bolted Two Diesels Together for the Sherman
America Had No Tank Engines in 1940 – So GM Bolted Two Diesels Together for the Sherman
Here is a question that nobody in the United States War Department wanted to answer in 1940.
You have designed a 30 ton tank.
You have designed a turret that traverses 360°.
You have designed suspension that can cross a plowed field at 20 mph.
You have armor plate.
You have a 75 mm cannon.
You have machine guns.
You have radios.
You have seats for a crew of five.
You have everything a modern medium tank requires except the one thing without which all of it is useless.
You have no engine.
Not no engine in the abstract, not a theoretical shortage.
You have no engine that exists in production in the quantities required with the horsepower, the torque, the reliability, and the physical dimensions necessary to fit inside the hull of the tank you have designed.
You have a country that builds more automobiles than the rest of the world combined that produces millions of internal combustion engines every year.
That employs hundreds of thousands of engineers and machinists and tool makers in factories stretching from Detroit to Los Angeles.
And not one of those factories has ever built a tank engine because the United States has never needed a tank engine.
The United States in 1940 barely has tanks.
This is the part of the story that embarrasses people, so they skip it.
In 1940, the United States Army possessed fewer than 400 tanks.
Most of them were obsolete.
The French Army, which would be destroyed in 6 weeks that spring, had more tanks than America.
The Polish Army, which had been destroyed the previous September, had fielded more modern armored vehicles than the entire American inventory.
Germany had over 2,000 tanks in its Panzer divisions.
Britain was producing tanks at an accelerating rate despite being under bombardment.
Even Italy had more tanks than the United States.
The reason was simple and depressing.
Congress had not funded tank development.
The peaceime army had been starved of money for two decades.
Between the wars, the army’s budget for armored vehicles was so small that commanders had to choose between buying tanks and buying ammunition to train with the tanks they already had.
They chose ammunition.
When they needed to practice armored tactics, they sometimes used trucks with cardboard signs that read tank taped to their sides.
The cavalry branch, which should have been developing armored doctrine, was forbidden by law from operating tanks because tanks were assigned to the infantry.
The National Defense Act of 1920 had explicitly placed tanks under infantry control, and the bureaucratic inertia of that decision persisted for 20 years.
The result was that when Germany demonstrated in Poland and France what a modern armored force could accomplish, the United States had almost nothing to respond with.
No tanks, no tank factories, no tank engines, no tank crews, and no institutional experience in any of the above.
The tank engines that did exist in the American military were aircraft engines.
Specifically, the Wright R975 Whirlwind, a 9 cylinder air cooled radial engine originally designed for biplanes in the late 1920s.
It was compact and reasonably powerful at 350 horsepower.
It ran on high octane aviation gasoline.
Continental Motors in Detroit held the license to build it for military ground vehicles, and they would eventually produce over 53,000 of them.
The R975 was not a bad engine, but it was not a tank engine.
It was an airplane engine jammed into a steel box and asked to do work for which it had never been designed.
The problems were fundamental.
The R975 was air cooled, which was ideal in an airplane where propeller blast and forward air speed provided constant air flow over the cylinder fins, but problematic in a tank where the engine sat in an enclosed compartment behind the crew.
A massive fan and shroud had to be attached to force air over the cylinders.
The engine was wide and tall because of its radial configuration, nine cylinders arranged in a circle like the petals of a flower, and this dictated the size of the entire rear hull.
It was underpowered for the weight of the tank and had to be run at high revolutions to get the vehicle moving, which made it noisy and increased wear.
It burned gasoline, which was volatile and flammable, a characteristic that would later earn the Sherman tank the grim nickname Ronson among some troops after the cigarette lighter whose advertising slogan was lights first time every time.
And most critically, every R975 that went into a tank was an R975 that did not go into an airplane.
And in 1940 and 1941, with the Army Air Corps expanding at a desperate rate, the competition for aircraft engines was fierce.
The War Department understood the problem.
They needed alternative engines.
They needed them immediately, and they turned to the only industry in America that had the engineering talent, the manufacturing capacity, and the institutional knowledge of internal combustion engines to produce them at the scale the war would demand.
They turned to Detroit.
The response from the American automobile industry was extraordinary, improvised, and occasionally insane.
Each manufacturer approached the problem differently, and each solution revealed something about the corporate personality of the company that produced it.
Chrysler, whose engineers had never designed a military engine, looked at the power requirement and the components they had available and arrived at a solution that borders on the surreal.
They took five six-cylinder flathead automobile engines, the kind that powered dodos and Plymouths on American highways, arranged them in a radial pattern around a central gear case like the pedals of an industrial flower, and bolted them together into a single 30cylinder monstrosity called the A57 multibank.
30 cylinders, five separate engines, five ignition systems, five sets of spark plugs, five carburetors, five cooling circuits.
It produced 370 horsepower, weighed more than any sane engineer would have accepted, and required mechanics who could essentially service five automobile engines simultaneously within the confined space of a tank’s engine compartment.
The US Army tested it and did not want to use it even for training.
It was only the willingness of the British to accept the A57p powered M4 A4 through lend lease that kept the multiank in production.
The British, desperate for tanks of any kind, received over 7,000 M4 A4s and pronounced them somewhat to everyone’s surprise reliable in service.
Ford took a different approach.
They had been developing a 27 L aluminum V12 aircraft engine before the war and their engineers realized that if they cut four cylinders off the design, the resulting V8 could produce the power and torque required for a tank while fitting within the hull dimensions of the Sherman.
The Ford GAAA displaced,00 in an astonishing figure that dwarfed any automobile engine of the era.
It produced 450 horsepower on gasoline and delivered its power smoothly and reliably.
After some initial teething problems were resolved, the GAA became the Army’s preferred tank engine, and the M4 A3 Sherman that carried it was designated the standard model for US Army use.
But the GAA was a purpose-built solution that took time to develop and debug.
And it was not available in quantity during the critical early years of the war when Shermans were needed by the thousands.
And General Motors did something that on its face seemed like the dumbest idea of all.
They took two bus engines and bolted them together.
The engine was called the GM6046.
It was in the most literal sense two General Motors 671 inline six-cylinder diesel engines placed side by side in a U configuration, each with its own crankshaft connected through separate clutches to a common output shaft that fed the tank’s transmission.
It was not a V12.
It was not a redesign.
It was two complete engines, each fully functional on its own.
Mechanically coupled and installed in the engine bay of a medium tank.
It displaced 850 cub in.
It weighed 5100 lb dry.
It produced 375 net horsepower and 1,000 ft-lb of torque.
And it would power the M4 A2 Sherman, one of the most widely produced and distributed tank variants of the Second World War.
To understand why anyone would build an engine this way, you need to understand the 671 diesel and the man whose obsession created it.
That man was Charles Franklin Ketering and his story intersects with the GM6046 at a point decades before the first Sherman rolled off the assembly line.
Ketering was born in 1876 on a farm in Lowdenville, Ohio.
He was by any measure one of the most prolific and consequential inventors in American industrial history.
He invented the electric starter for automobiles, eliminating the hand crank that had broken wrists and killed people.
He developed the battery coil ignition system that made modern gasoline engines practical.
He co-founded Delo, which became the electrical division of General Motors.
He developed leaded gasoline which solved the problem of engine knock and dramatically increased power output.
Though the long-term health consequences would haunt his legacy.
He developed Freon, the refrigerant that made modern air conditioning possible.
And he was obsessed with diesel engines.
Ketering’s interest in diesels began around 1913 when he first experimented with compression ignition engines for generator sets.
By 1928, he had purchased a diesel-powered yacht specifically so he could spend time in the engine room studying and tinkering with the motors.
Alfred Sloan, the president of General Motors, later wrote that in the matter of diesel engines at GM, Charles Ketering comes very close to being the whole story.
The GM research laboratories had been testing diesel designs since 1921, but it was Ketaring who pushed relentlessly for a new kind of diesel, one that was lighter, more compact, and more powerful than the massive, slow turning industrial engines that characterized the technology at the time.
The breakthrough was the two-stroke cycle.
Most diesel engines then, as now, operated on a four- stroke cycle.
Intake, compression, power, exhaust, four strokes of the piston for every one power stroke.
Ketering and his engineers working with the research division and later with engineers brought over from the Winton engine company that GM had acquired in 1930 developed a two-stroke diesel that fired on every revolution of the crankshaft rather than every other revolution.
This meant that cylinder forcylinder, the engine could theoretically produce nearly twice the power of a comparable four- stroke.
In practice, the gains were somewhat less than double, but they were substantial.
A relatively small, lightweight engine could produce horsepower figures that previously required a much larger and heavier machine.
The result was the Detroit Diesel Series 71, and the flagship product was the 671.
Six cylinders in a line, 71 cubic inches per cylinder, 4 and a/4 inch bore, 5 in stroke, total displacement of 426 cubic in.
It used a roots type supercharger, not for boost in the traditional sense, but because a two-stroke diesel cannot naturally aspirate the way a four- stroke can.
In a four-stroke engine, the piston’s downward intake stroke creates a vacuum that draws fresh air into the cylinder.
A two-stroke has no dedicated intake stroke.
The fresh air charge must be forced into the cylinder by external means.
The roots blower, a pair of meshing loes driven by gears off the crankshaft, provided this forced air supply.
It pressurized the air slightly above atmospheric pressure and pushed it through cord passages in the engine block into ports machined into the cylinder walls.
When the piston descended past these ports, the pressurized air rushed in, scavenging the spent combustion gases that were simultaneously exiting through conventional puppet valves in the cylinder head.
The exhaust valves pushed open by push rods and rocker arms driven by a geardriven camshaft allowed the hot gases to escape upward while the fresh air entered from the sides.
It was an elegant system, simple in concept, but requiring precise timing of port opening, valve opening, and fuel injection to function efficiently.
The injection system was unit type, one injector per cylinder, with fuel pressurized only within the injector body itself, eliminating the complex high-pressure fuel lines that ran from a central injection pump to individual cylinders in other diesel designs.
Those high-pressure lines were a constant maintenance headache in other engines, prone to cracking, leaking, and developing fatigue failures from the pulsing pressure cycles.
The 671’s unit injectors delivered fuel precisely and eliminated an entire system of potential failures.
The engine was what GM called symmetrical, meaning the blower, exhaust manifold, starter, water pump, and accessories could be mounted on either side of the block to suit different installations.
A feature that would prove invaluable when the engine was adapted for dozens of different military and civilian applications.
The 671 entered production in January 1938 when the newly formed Detroit diesel engine division of General Motors began manufacturing at their facility in Detroit.
The first year saw modest numbers.
Almost 700 engines were delivered to GM’s coach and truck division for installation in city transit buses.
And the Gray Marine Motor Company began producing marinized versions for commercial boat use.
Tuned for commercial bus duty, the engine produced 165 horsepower at a govern speed designed for the stopand go cycles of urban transit.
In navy rated military configuration with the governor adjusted and the blower set for higher output, it produced 225 horsepower.
It was not a powerful engine by the standards of the tank designers who needed 400 horsepower, but it was a remarkable engine by the standards of diesel technology in 1938.
Diesels of that era were massive, slow turning, heavy machines suited for ships and power stations.
The 671 was compact, relatively lightweight for a diesel, surprisingly responsive in its throttle behavior, and built with the manufacturing philosophy of General Motors, which meant interchangeable parts, standardized components, and the ability to scale production from hundreds to tens of thousands when the need arose.
When the War Department came to General Motors in 1941 and said, “We need a tank engine.”
The engineers at Detroit Diesel looked at the requirement and looked at what they had.
They needed 375 horsepower minimum.
A single 671 produced 225 at military rating.
Two of them would produce 450.
The math was obvious.
The engineering was less obvious, but entirely achievable.
They placed two 671 engines side by side running in the same direction and coupled their output shafts to a common transfer case that drove a single propeller shaft to the tank’s transmission.
Each engine retained its own crankshaft, its own blower, its own cooling system, its own fuel injectors, and its own clutch.
The two engines were separately clutchable, which meant that if one engine was damaged in combat, the crew could disconnect it and limp home on the surviving engine at reduced power.
This was not a theoretical advantage.
Tank crews in the field reported using single engine operation to reach repair depots after taking hits that would have immobilized a single engine tank.
The GM6046 was first used in the M3 A3 and M3A5 variants of the M3 Lee medium tank and then adapted for the M4 A2 Sherman.
The Pilot M4 A2 was ready for testing by April 1942, the same month that Fisher Body’s tank arsenal and Pullman Standard Car Company began production.
In trials, the M4 A2 outperformed both the standard M4 and the M4A1 with the R975.
The diesel engine’s torque characteristics were superior at low speeds, giving the tank better control during slow speed maneuvering, hill climbing, and the kind of deliberate tactical movement that characterizes actual combat rather than high-speed road marches.
Previous experience with the diesel installation in the M3A3 and M3A5 meant that production could be ramped up quickly.
A total of 10,968 M4 A2 Shermans were built across six factories between April 1942 and July 1945.
But here is where the story takes a turn that the people who designed the GM646 could not have predicted.
The US Army did not want it.
The decision was made by Lieutenant General Jacob L. Devers, Chief of the Army’s armored force.
Devs ordered that no diesel engine Shermans be used by the army outside the continental United States.
The reasoning was logistical.
The army ran on gasoline.
Its trucks ran on gasoline.
Its jeeps ran on gasoline.
Its halftracks and armored cars and self-propelled guns ran on gasoline.
The supply chain was built around gasoline.
Adding diesel fuel to the supply chain would create a second fuel requirement, complicating an already staggeringly complex logistics system that had to deliver millions of gallons of fuel across oceans and continents to vehicles fighting on multiple fronts simultaneously.
The Army was not willing to accept that complication.
The M4 A2 was relegated to training use within the United States.
The army would fight with the gasoline powered R975 Shermans initially and later with the Ford GAAA powered M4 A3 which became the preferred model by late 1944.
This decision would have been a death sentence for the GM6046 if the United States had been fighting alone.
But the United States was not fighting alone.
It was supplying half the world.
The M4 A2 became the primary Sherman variant allocated to the Lend lease program and the nation that wanted it most was the Soviet Union.
The Red Army ran on diesel.
The T34, the backbone of Soviet armored forces, was powered by the V2 diesel engine.
Soviet logistics were built around diesel fuel.
Soviet mechanics understood diesel maintenance.
A gasoline powered tank was useless to them, not because of any deficiency in the vehicle, but because it required a fuel that the Soviet supply system did not stock in the quantities necessary to keep an armored formation in combat.
When the first Lendle M3 Lee tanks arrived in the Soviet Union running on gasoline, the Soviets used them but complained bitterly about the fuel incompatibility.
When they were offered the M4 A2 with its GM6046 diesel, they embraced it.
The commander of the Soviet tank force specifically requested that all future deliveries be M4 A2 Shermans.
No gasoline tanks, only diesel.
The United States shipped 4,12 M4 A2 Shermans to the Soviet Union between 1943 and 1945.
Of these, 2007 carried the original 75mm gun and 295 mounted the more powerful 76 mm.
They traveled by two routes.
The Arctic convoys carried them through the Norwegian Sea to the ports of Merman and Archangel, running a gauntlet of German Ubot, aircraft, and surface raiders.
The Pacific route carried them from the American West Coast across the Bearing Straight to Vladivosto, a longer but safer journey after 1943.
Not all of them arrived.
Some went to the bottom of the Arctic Ocean, torpedoed in the cold water, their GM 6046 engines flooding and seizing in the dark.
The Soviets called the Sherman Mcha, a phonetic rendering of the Russian pronunciation of M4, Michir.
They gave the M4 A2 to their best units.
Guards formations, the elite of the Red Army, units that had earned their designation through extreme heroism and strategic importance, were preferentially equipped with Shermans.
The first guard’s mechanized corps, the third guard’s mechanized corps, the ninth guard’s mechanized corps, the sixth guard’s tank army.
These were not second line formations receiving castoff equipment.
These were spearhead units, the tip of the Soviet offensive that would drive from Kursk to Berlin.
The decision to give them Shermans rather than T-34s was deliberate and telling, and it reflected a battlefield calculus, but the soldiers themselves understood.
Soviet tank crews who operated the M4 A2 left accounts that read today sound almost affectionate.
They appreciated specific characteristics that the T-34, for all its legendary toughness, did not offer.
The Sherman was quieter.
Its rubberized tracks produced less noise on hard ground than the all steel tracks of the T34, which clattered and clanked with every meter of movement and announced the approach of an armored column from a mile away.
The GM6046 being a diesel was quieter than a gasoline engine at equivalent power and the ability to disconnect one engine and run on a single unit at low power made the tank nearly silent during night approach marches.
Soviet commanders used this feature tactically infiltrating Sherman equipped formations through gaps in German lines under cover of darkness.
Their approach masked by the quiet rumble of a single 671 idling at low revolutions.
The Sherman had superior gun sights, giving the gunner better optics for target acquisition at ranges where the T34’s cruder sits lost definition.
It had more advanced radio equipment, a critical advantage in the fastmoving combined arms operations that characterized the deep battle doctrine of the late war Soviet army.
It had an auxiliary power unit that could charge the batteries without running the main engine, a feature the T-34 lacked, meaning Soviet T34 crews had to idle their main engine whenever they needed electrical power for the radio or turret traverse motor, consuming fuel and revealing their position with engine noise and exhaust.
And the Sherman was by multiple Soviet accounts more reliable and easier to maintain than the T-34.
A claim that would surprise Western historians who had absorbed the post-war narrative that Soviet tanks were crude but indestructible.
The truth was more nuanced.
The T-34 was a magnificent design produced under conditions of such desperate haste that quality control was inconsistent.
The Sherman was a less revolutionary design produced with American manufacturing precision.
And in terms of day-to-day reliability, the American product was often superior.
The Soviets rejected alternatives.
When offered the M4 A4 with its Chrysler A57 multibank engine for evaluation, they tested it on the proving grounds, received lectures on its maintenance requirements, and declared it too complicated.
Five automobile engines joined together meant five times the maintenance, five times the potential failure points, five times the number of components that could fail in the Russian winter when metal became brittle and lubricants thickened.
They wanted the GM6046 and nothing else.
Two diesel engines they understood.
Five gasoline engines they did not trust.
M4 A2 Shermans of the Sixth Guard’s Tank Army fought their way into Berlin in April 1945.
The GM6046 engines that had been designed for American city buses now powering steel behemoths through the rubble of the capital of the Third Reich.
The photograph of a Sherman with a Soviet red star painted on the turret parked in front of the Reichto is an image that captures the absurd internationalism of the American industrial war effort.
An engine designed by Charles Ketering in Detroit.
Built from two bus motors bolted together in a factory in Michigan.
Installed in a tank built by fisher body, shipped across the Pacific Ocean to Vladivvastto, driven across Siberia on rail cars and finally sent into battle by the Red Army in the streets of Berlin.
The engine never knew what country it was fighting for.
It just ran.
The other major American combat user of the M4 A2 was the United States Marine Corps, and the logic was identical to the Soviet logic, though the geography was different.
The Marines fought in the Pacific.
They arrived on beaches in landing craft.
Those landing craft were powered by Detroit diesel 671 engines.
Approximately 57,000 individual 671 engines were used on American landing craft during the war, including 19,000 on the iconic LCVP Higgins boats that delivered troops to the beaches of Tarawa, Saipon, Ewima, and Okinawa.
And 8,000 on LCM Mark III mechanized landing craft.
Having a tank that burned the same fuel as the landing craft that carried it to the beach simplified logistics enormously.
A Marine division fighting on a Pacific island did not have the luxury of a sprawling continental supply chain.
It had what was on the beach and what the Navy could bring in.
If the tanks and the boats drank the same fuel, the logistical problem was hald.
Marine M4 A2s fought at Cape Gloucester, at Saipon, at Guam, at Pelu, at Ewoima, at Okinawa.
On those volcanic islands where the enemy fought from tunnels and bunkers and coral caves, the Sherman was used as close infantry support, crawling forward at walking pace to blast Japanese positions with its 75 mm gun and bow machine gun, while infantry sheltered behind its hull and turret.
The tank was not a cavalry weapon in the Pacific.
It was a siege weapon.
It advanced 10 yards at a time, stopped, fired, advanced again.
The engine idled for hours at a time, creeping forward in first gear over terrain that would have destroyed a lesser drivetrain.
The diesel engine had multiple practical advantages in this role.
The obvious one was logistics.
Diesel fuel has a higher flash point than gasoline.
It does not ignite as readily when the tank is hit by enemy fire.
A gasoline powered Sherman that took a penetrating hit from an anti-tank gun could erupt in flame as the fuel atomized and met the hot metal of the engine block, the exhaust manifold, or a spark from severed electrical wiring.
The crew had seconds to bail out before the ammunition began cooking off.
A diesel-powered Sherman was less likely to burn.
The fuel would not ignite from a spark.
It required sustained heat and atomization to combust.
This difference was not absolute.
A diesel tank could still burn if the hit was catastrophic enough.
if the fuel sprayed onto a surface hot enough to reach diesel’s ignition point.
But the margin of safety was real, and it was meaningful to five young men sitting inside a steel box that Japanese soldiers were trying to destroy with artillery, anti-tank guns, magnetic mines strapped to poles, and suicide charges.
The second advantage was durability.
The two-stroke 671 diesel was mechanically simpler than any of the gasoline alternatives.
It had no ignition system, no spark plugs, no distributor, no ignition wires, no carburetor.
Diesel combustion is initiated by the heat of compression alone.
Fuel is injected directly into the cylinder at the top of the compression stroke and the compressed air heated to over a,000° F ignites it without any electrical assistance.
This meant that an entire category of failure modes that plagued gasoline engines, fouled spark plugs, cracked distributor caps, failed ignition coils, flooded carburetors simply did not exist.
In the humid, saltladen, sandchoked environment of a Pacific island where every piece of equipment was under assault from the climate as much as from the enemy, this simplicity translated directly into reliability.
The third advantage was fuel economy.
The diesel cycle is inherently more thermally efficient than the gasoline auto cycle because it operates at a higher compression ratio.
The GM6046 with a compression ratio of 16:1 compared to the R975’s 5.7:1 extracted more useful energy from each gallon of fuel.
This gave the M4 A2 a longer operational range on the same volume of fuel, a meaningful advantage on islands where fuel had to be brought ashore in drums and stacked on beaches under enemy fire.
The GM6046 also inherited a property of its parent engine that proved surprisingly useful.
The Soviet Union copied it.
After the war, Soviet engineers reverse engineered the Detroit diesel 671 from Lendley’s examples and produced their own version, designated the Yaz 204 for the four-cylinder variant and the Yaz 206 for the six-cylinder.
These Soviet copies were manufactured at the Yaruslav automobile plant and installed in heavy trucks that served the Soviet military and civilian economy for decades.
The Krazheavy trucks that carried Soviet missiles and military equipment through the Cold War were in their engine rooms descendants of the bus motor that Charles Kettering team had developed in Detroit in the 1930s.
And what of the engine itself, the 671, the parent of the GM 6046?
It became one of the most successful and longived diesel engines in history.
Production continued from 1938 to 1995, 57 years of continuous manufacturer.
The engine that powered Sherman tanks and Higgins boats went on to power tugboats, fishing vessels, construction equipment, generators, fire trucks, municipal buses, long haul trucks, and industrial machinery on every continent.
Detroit Diesel built over 5 million engines since 1938.
Almost half a million series 71 engines are still in service worldwide.
The nickname given to them by the people who worked with them, the screaming Jimmy, derives from the distinctive high-pitched wine of the two-stroke diesel at speed, a sound that anyone who is stood near a working tugboat or an older Greyhound bus would recognize instantly.
The sound is distinctive because the two-stroke cycle produces a power stroke on every revolution rather than every other revolution, making the engine sound like it is running at twice its actual speed.
A 671 turning at 2,000 RPM sounds like a gasoline engine turning at 4,000.
It is a mechanical scream, urgent and relentless, and it is the sound that carried Sherman tanks across the steps of Ukraine and the beaches of the Pacific and the streets of Berlin.
There is an irony buried in the story of the GM6046 that deserves to be spoken aloud.
The US Army, which made the decision about which engines to use in combat, rejected the diesel.
They chose gasoline.
Their reasoning was logical, defensible, and probably correct from a pure logistics standpoint.
The American supply chain in the European theater of operations was already the most complex logistical operation in human history.
The Red Ball Express alone, the truck convoy system that fed Patton’s Third Army and the other Allied forces racing across France after D-Day, consumed millions of gallons of gasoline per week.
Adding a parallel diesel supply chain would have required separate storage, separate delivery trucks, separate depot management, and the constant risk that the wrong fuel would be delivered to the wrong unit at the wrong time.
In a theater where entire armored divisions could be immobilized by a fuel shortage, the decision to standardize on gasoline was not stupid.
It was rational.
But the consequence was that the diesel-powered Shermans, the ones with the reliable, torquy, fuelefficient, harder to ignite GM6046, were given away.
Sent to the Soviets, who put them in their best formations.
Sent to the British who received over 5,000 M4 A2s and designated them Sherman 3, deploying them in North Africa, in Italy, in Normandy, in the long slog across the low countries and into Germany.
Sent to the Marines, who had not asked for diesel tanks specifically, but were grateful for the fuel commonality with their landing craft.
Sent to the free French, who used them to liberate Paris and drive through Alsace.
Sent to the Poles, who fought in them at Normandy and across the Netherlands.
The army kept the gasoline tanks and gave the diesels to everyone else.
And the everyone else fought in them from Elamagne to Berlin to Okinawa.
The Soviets liked the M4 A2 enough to reject the M4 A4 when it was offered to them.
They tested the Chrysler A57 multi-bank engine that powered the M4 A4 and declared it too complicated to maintain in the field.
They wanted the GM6046 and nothing else.
The British, who received more M4 A2 Shermans than any other Lend lease partner, used them in North Africa, in Italy, in Normandy, and across northwestern Europe.
The diesel Sherman fought on every front of the war except the one where the American army was fighting because the American army had decided it did not want it.
49,234 Sherman tanks were built during the war, making it the second most produced tank in history behind only the Soviet T34.
They were manufactured by 11 different companies from locomotive builders to car body stampers in factories scattered across the industrial heartland of America.
And they were powered by four different engines from three different manufacturers.
Each engine a product of desperation, ingenuity, and the willingness of American industry to solve problems that should have been solved 20 years earlier, but were not because nobody had been willing to pay for the solution.
The R975 was an airplane engine in a tank.
The A57 was five car engines in a circle.
The Ford GA was an aircraft V12 with four cylinders cut off.
And the GM6046 was two bus engines bolted side by side.
None of them were purpose-built tank engines.
Every one of them was a lashup, an improvisation, a what do we have that might work solution assembled by engineers who did not have the luxury of time or the option of failure.
The Germans, by contrast, had purpose-built tank engines.
The Maybach HL230 that powered the Panther and the Tiger was designed from the outset as a tank power plant.
It was beautifully engineered, technically sophisticated, and produced in numbers that were a fraction of what the American factories turned out.
Germany built about 6,000 Panthers during the entire war.
America built 49,000 Shermans.
The Maybach was a better engine.
The GM 6046 and its siblings were more engines.
And in a war of attrition, of industrial output, of logistics measured in millions of tons and thousands of miles, more was what won.
The wartime production numbers for the 671 alone tell the story of American industrial scale.
In 1941, GM built 9,671 engines.
By 1944, annual production had reached 62,000 units.
Over 100,000 GM diesel engines were built during the war, counting both the military and grey marine production.
22,000 of those went into M4 A2 Shermans as twinpacked 6046 units.
11,000 went into M10 tank destroyers.
57,000 individual 671s went into landing craft.
4,000 went into Canadian-built Valentine tanks sent to the Soviet Union.
The engine that had been designed for city buses ended the war having powered a significant fraction of every Allied vehicle that floated, rolled, or crawled into battle.
Charles Kettering died in 1958.
He held over 186 patents.
He had transformed the American automobile, helped create the modern diesel engine, and set in motion the industrial chain that put two of his bus engines inside a tank that fought on three continents.
He never designed a tank engine.
Nobody asked him to.
He designed a two-stroke diesel for buses and trucks and boats.
And when the world caught fire, his engineers doubled it and dropped it into a Shoreman.
And it worked.
The GM6046 is not in any hall of fame.
There is no monument to it.
It is not celebrated the way the Merlin engine is celebrated or the way the 34’s V2 diesel is celebrated or even the way the Ford GAA is remembered by enthusiasts who admire its brute displacement and raw power.
The 6046 was the workhorse that nobody chose first.
It was the engine that the army rejected and the world embraced.
It was two of something that already existed, joined together not by inspiration, but by necessity, and sent to war in the hands of Marines and Soviets and Frenchmen and Britons, who did not care what it looked like or how it was designed, only that it started every morning and ran all day and did not catch fire when somebody shot at it.
That was enough.
