The Merlin Made in America: How Packard’s Engineers Turned a Hand-Built British Marvel Into the Mass-Produced Powerhouse That Won the Air War

On August 2, 1941, inside the Packard Motor Car Company’s massive East Grand Boulevard plant in Detroit, Michigan, two engines roared to life. Their sound echoed off steel beams and concrete floors, a deep mechanical thunder that shook the test stands. At first glance they looked like Rolls-Royce Merlin engines—sleek, purposeful machines destined for fighter aircraft over Britain.

But these engines were different.

They were American copies of the British Merlin, constructed not by the craftsmen of Derby, Crewe, or Glasgow, but by Packard engineers who had spent nearly a year redrawing every blueprint, re-measuring every dimension, and reinventing every manufacturing step.

The witnesses gathered that day had no idea they were watching something revolutionary. Because hidden behind the noise was a secret that would alter the balance of air power in the Second World War:

Detroit had just turned a hand-fitted, artisan-built British masterpiece into an American mass-production engine.

It was the beginning of a story that would culminate in the P-51 Mustang escorting American bombers all the way to Berlin—and back.

A Fighter Without a Future—Until It Found a New Heart

By early 1942, the United States had a problem. The Army Air Forces possessed a promising new fighter—the P-51 Mustang. It was fast, agile, and beautifully designed. At low altitude, it outperformed almost anything in the sky.

But climb higher than 15,000 feet, and it began to gasp.

The reason was simple: the Allison V-1710 engine. Its single-stage supercharger couldn’t compress enough thin, cold air at altitude. Above 25,000 feet, it produced barely 1,000 horsepower. At 30,000 feet—where German fighters prowled—the Allison-powered Mustang struggled to breathe at all.

Meanwhile, American heavy bombers—B-17s and B-24s—flew at those exact altitudes as they crossed into Germany. Without long-range escort fighters capable of matching their cruise height, losses mounted.

During the August 1943 raid on Schweinfurt, the Luftwaffe destroyed 60 bombers in a single day.

The need was clear:
America didn’t require a new fighter. It needed a new engine.

Across the Atlantic, Britain already had one.

The Merlin: Brilliant, Powerful—But Impossible to Mass-Produce

The Rolls-Royce Merlin was a masterpiece of British engineering. Its two-stage, two-speed supercharger—designed by aerodynamicist Stanley Hooker—allowed it to maintain sea-level manifold pressure all the way up to 30,000 feet. It was the engine that let Spitfires and Hurricanes climb rapidly to meet German raiders during the Battle of Britain.

But the Merlin had a flaw:
it was built almost entirely by hand.

Each engine contained 14,000 individually fitted parts. Bearings were matched by skilled craftsmen. Cylinder heads were adjusted by hand. Supercharger impellers were filed and balanced by artisans who used experience rather than formulas. Even the fasteners were unique, using Britain’s intricate Whitworth thread system.

Rolls-Royce factories were working around the clock. Yet by late 1940, even with new “shadow factories,” Britain could build only about 200 Merlins per week.

It needed ten times that.

And so, London turned to Detroit.

Packard Steps In—And Realizes the Task Is Herculean

Packard Motor Car Company, known for its luxury automobiles, was not an obvious choice to build aircraft engines. But Packard had a reputation for engineering excellence—and, more importantly, for precision manufacturing at scale.

In September 1940, British engineers arrived in Detroit with crated Merlin engines, stacks of blueprints, and a sense of urgency.

Packard engineers immediately recognized the enormity of the challenge.

Problem 1: Incompatible Measurement Systems

Rolls-Royce drawings used an archaic mixture of imperial units and British engineering standards that pre-dated modern measurement conventions. Converting them to American standards wasn’t simple arithmetic—it required understanding the purpose behind each dimension.

Problem 2: British Whitworth Threads

Every bolt, nut, and screw used a 55-degree thread profile with rounded crests and roots—totally incompatible with American tooling. Packard would have to manufacture every threaded fastener in-house from scratch.

Problem 3: Merlin Tolerances Were Designed for Hand-Fitting

Rolls-Royce allowed wide manufacturing tolerances because craftsmen would hand-adjust each part during assembly. American mass production allowed no such luxury.

Packard engineers had to replicate the Merlin exactly—every contour, every dimension, every fastener—while making each part so precisely that any component could fit any engine without adjustment.

Many British engineers doubted it could be done.

But Packard had an answer that stunned them:

“Your tolerances are too loose for us.”

The Reinvention of a Legend

From late 1940 through mid-1941, Packard engineers embarked on one of the greatest technical translation projects in industrial history.

Over 6,000 engineering drawings were recreated—line by line, dimension by dimension—not because the British versions were wrong, but because they weren’t compatible with mass production.

Packard:

tightened tolerances

re-designed manufacturing fixtures

re-envisioned tooling

replaced hand-fit surfaces with precision-machined ones

and ensured perfect interchangeability of all components

They kept every performance spec intact.

But they redesigned the method behind every part.

A Metallurgical Breakthrough

Rolls-Royce bearings used copper-lead alloys that required careful break-in.

Packard developed a silver-lead alloy with indium plating that reduced friction and extended lifespan. Rolls-Royce was skeptical—until testing proved Packard’s bearings were better.

They adopted the American design.

Solving the Threading Nightmare

Instead of switching to American screw standards (which would have made parts incompatible), Packard:

designed and built American-made Whitworth tooling

created thread gauges never before used in the U.S.

trained machinists in British thread forms

The result:
Detroit-built Merlins used British fasteners made to tighter tolerances than the originals.

The Supercharger Challenge

The Merlin’s two-stage supercharger impellers spun at over 30,000 rpm. Rolls-Royce balanced them by hand.

Packard introduced:

precision casting

automated machining

dynamic spin-balancing equipment

Impellers emerged from Packard machines nearly perfect—with minimal hand work required.

The Cooling Puzzle

Packard redesigned coolant passages to improve flow and simplify production without altering temperature performance.

Every subsystem was examined. Every part improved for manufacturability.

August 1941: The First Packard Merlin Runs

After just eleven months of translation, redesign, and re-engineering, Packard tested its first Merlin—the V-1650-1—on August 2, 1941.

It met every Rolls-Royce performance requirement.

When word reached Prime Minister Winston Churchill, he reportedly wept.

Britain would have the engines it needed.

But the true revolution had just begun.

Detroit Goes to War: Mass Production at Scale

Packard transformed its Detroit plant into a marvel of wartime manufacturing. The Merlin assembly line used:

dedicated machining cells

statistical process control

rigorous quality inspection

standardized tooling

interchangeable parts

By 1943, Packard was producing more Merlins per week than Rolls-Royce’s factories combined.

At peak output, Packard built over 400 engines per week.

The Engine That Transformed the P-51 Mustang

The Packard V-1650-3 and V-1650-7 variants—based on Rolls-Royce’s two-stage Merlin 60-series—unlocked the Mustang’s full potential.

With the new engine, the P-51 gained:

sustained performance at 30,000–40,000 feet

1,300+ horsepower at altitude

437 mph top speed

the range to escort bombers from England to Berlin

The Allison Mustang had struggled to climb to bomber altitude. The Packard Merlin Mustang could:

outclimb

out-dive

out-run

and out-fight almost every German aircraft

The Luftwaffe quickly learned to fear the sound of Merlins at high altitude.

By the end of the war, Packard had built 55,523 Merlin engines—more than the entire Rolls-Royce wartime output.

A Legacy That Outlived the War

Packard’s reinvention of the Merlin changed far more than one aircraft.

1. Precision Mass Production Becomes the Future

The methods Packard developed—tight statistical controls, advanced casting, interchangeable parts—became foundational in aerospace manufacturing.

Modern jet engines still use descendants of Packard’s wartime techniques.

2. Bearing Technology Takes a Leap Forward

The silver-lead-indium bearings Packard pioneered influenced U.S. automotive engineering for decades.

3. Thread Standardization Becomes a Global Priority

The effort required to maintain British and American thread compatibility helped inspire the eventual creation of international standards.

4. Transatlantic Collaboration Set the Tone for Future Alliances

Packard didn’t replace Rolls-Royce engineering—it honored it, translated it, and elevated it.

Rolls-Royce, in turn, adopted Packard improvements.

This spirit of mutual respect shaped post-war aerospace cooperation.

The Sound of Two Nations Working as One

Today, when a restored P-51 Mustang streaks across an airfield and the deep, throaty howl of its Merlin engine fills the air, you are hearing more than mechanical noise.

You are hearing:

British innovation

American manufacturing mastery

a partnership that spanned oceans

an engine built by hand in Derby and perfected by system in Detroit

You are hearing a piece of history that helped win the air war over Europe.

A fighter is only as good as its engine.
And the story of the Merlin—designed in Britain, reinvented in America—is a reminder that engineering brilliance does not belong to any one nation.

It belongs to those willing to solve hard problems under impossible timelines.

Packard solved 14,000 of them.

And the world changed because of it.