Harley’s Screamin’ Eagle Milwaukee-Eight 135 Is a Sleeper Swap Engine Waiting to Happen

The crate engine world is dominated by the same tired suspects: LS small blocks, Coyote V8s, and whatever K-Series motor is currently hot on the tuning forums. But buried in Harley-Davidson’s factory performance catalog is something far more interesting—a sleeper swap candidate that nobody’s talking about. The Screamin’ Eagle Milwaukee-Eight 135 Stage IV is a 2.2-liter V-twin that officially lives in Harley touring motorcycles from 2021 to 2024, but its real potential lies elsewhere: in the engine bays of cars that weigh less than a stack of encyclopedias.

This isn’t revolutionary thinking—motorcycle engines in cars have a long pedigree. But what makes Harley’s 135 cubic inch (2,212cc) monster intriguing is its torque personality. Rated at 143 pound-feet, with most of that grunt arriving early in the rev range, this engine doesn’t behave like a typical V-twin. It hits like a sledgehammer instead of winding up to a frantic wail. That’s the opposite of a Hayabusa engine (the classic go-to for tiny car swaps), which demands you chase the tachometer needle into the stratosphere.

The Engine: Serious Hardware Hiding in Plain Sight

The Milwaukee-Eight 135 packs serious internals for a motorcycle powerplant. A 4.31-inch bore paired with a 4.63-inch stroke creates that monster displacement. Add high-lift cams, high-performance pistons, and a 68-millimeter throttle body, and you’ve got an engine that’s engineered to inhale massive amounts of air. The 130-horsepower figure might seem quaint compared to a modern turbo-four, but horsepower tells only half the story—the torque delivery is where this engine separates itself from the poseur crowd.

In the motorcycle world, 2.2 liters is legitimately massive. For perspective, most modern motorcycles displace less than 1.0 liter. That’s why Harley originally intended this as a drop-in replacement for its heavyweight touring bikes. But that’s also precisely why some enterprising builder will inevitably shoehorn it into something ridiculous and make automotive history.

Where This Engine Actually Belongs: Tiny Cars and Absurd Ideas

The first rule of motorcycle engine swaps is non-negotiable: don’t waste displacement on heavy iron. This engine demands a lightweight chassis to sing. The obvious starting point is the Morgan 3 Wheeler, a car so magnificently unhinged that it practically designed itself around a motorcycle engine. Morgan actually offers a turbocharged Ford three-cylinder in the modern Super 3, which is sensible. Boring, even. But mentally swap that for a torque-monster V-twin, and you’ve got a car that would genuinely terrify insurance companies and occupants alike.

Japanese kei cars are the real golden ticket. A Honda Beat or Suzuki Cappuccino originally limped along with 660cc motors that felt criminally inadequate on an American freeway. Picture instead a Cappuccino—already a rear-wheel-drive, wedge-shaped slice of perfection—with 130 horsepower and 143 pound-feet of low-end torque. That’s more than triple the original displacement, delivered by an engine that actually has character. The Autozam AZ-1, that gull-winged micro sports car that looks like a Ferrari’s illegitimate child, deserves this treatment even more. Its styling promises drama its original engine could never deliver.

Classic British and Italian microcars round out the wish list. The original Fiat 500 and Austin Mini both have DIY engine swap lineages, though most swaps lean toward Hayabusa motors for the high-rpm wail. A Screamin’ Eagle 135 in either car would be a completely different animal—a low-slung, historically appropriate chassis paired with an engine that rewards torque management over horsepower chasing.

The Reality Check: Why This Isn’t a Weekend Project

Let’s not pretend this is simple. Motorcycle-to-car engine swaps are engineering nightmares wearing a cool mask, and the Milwaukee-Eight 135 brings its own special Hell with it. The first problem is packaging. This is a tall engine. Compared to a compact four-cylinder, the massive stroke length means the piston travel path is nearly vertical. The 45-degree V configuration adds to the headroom issue. If you want this motor under the hood of a low-slung sports car, you’re probably cutting a hole in the hood. Which, frankly, would look amazing—a V-twin poking through the bonnet would be the kind of visual statement no LS swap could match.

The second hurdle is transmission adaptation, and it’s a legitimate bear. Harley uses a separate primary drive and transmission case, not the integrated gearbox you’d find on a sportbike. Your options are ugly: either adapt the Harley 6-speed with a custom driveshaft linkage and reversing gearbox, or machine a bespoke adapter plate to bolt a traditional car transmission to the crank. Companies like Quaife make driveline components for bike-engined cars, but they’ll cost you. A Milwaukee-Eight adapter would require full custom fabrication from a shop that actually knows what it’s doing.

The Lunacy Beyond Cars

If a 135-powered kei car feels too restrained, the possibilities spiral into beautiful madness. Picture a trophy kart carrying 143 pound-feet of low-end torque—enough to test your grip strength on the steering wheel. A sand buggy with this engine would be absurd. Or really lean into the chaos: a drift trike, a riding lawn mower, a purpose-built go-kart that accidentally becomes faster than 90 percent of cars at a local track.

The motorcycle engine swap game has historically been dominated by Japanese sportbike mills—Hayabusas, CBR motors, GSX-R engines—because they’re readily available and bolt into small cars with enough determination and fabrication skill. But the Screamin’ Eagle 135 represents a different philosophy: American muscle delivered through unusual channels, a torque-rich experience instead of an RPM chase. That’s genuinely interesting territory.

Harley will keep selling these engines to touring bike owners, and that’s fine. But somewhere, in a garage not unlike yours, someone is already sketching the dimensions of a Suzuki Cappuccino with this V-twin nestled in the bay. And they’re probably right to do it.

Sources: Jalopnik