The Harley-Davidson Biggest Displacement Motorcycle: Every Record, Explained

Ask a group of Harley riders which bike has the biggest engine and you'll get three different answers, and all three of them are right. It just depends on whether you mean the biggest engine fitted to a regular model, the biggest ever bolted in from the factory, or the biggest crate motor Harley has ever built. Sorting out the Harley-Davidson biggest displacement story means keeping those three categories straight.

Here's the short version, then the details. The biggest regular-production engine across the lineup is the Milwaukee-Eight 121. The biggest engine ever factory-installed in a production model is the Screamin' Eagle 131, wrapped inside the limited CVO Road Glide RR. And the biggest street-compliant engine the Motor Company has ever created is the Screamin' Eagle 135, sold as a crate engine rather than fitted to a stock bike.

Understanding the difference between those three is the real story.

Why "displacement" matters before we talk numbers

Before getting into cubic inches, it helps to think about what displacement actually is.

Displacement is the total volume your engine's cylinders sweep as the pistons move from top to bottom. In plain terms, it's a rough measure of how much air-and-fuel mix the motor can draw in per revolution. Think of it as the size of the engine's lungs. All else being equal, bigger lungs mean the engine can gulp more and make more torque, which is exactly what you want when you're hauling a 800-pound touring bike down the highway with luggage and a passenger.

Harley measures engine size the traditional American way, in cubic inches. One cubic inch is roughly 16.4 cubic centimeters, so a 121-cubic-inch V-twin works out to about 1,977 cc. If you're used to metric, every Harley engine size below is easy to convert.

The largest Harley-Davidson production engine: 121 cubic inches

Harley's current engine family is the Milwaukee-Eight, the ninth-generation "big twin" V-twin. According to Harley's official engine history, it debuted in 2016 and its displacement has grown steadily since.

The family breaks down like this:

• 107 (1,753 cc). The entry point in the Milwaukee-Eight line.

• 114 (right around 1,868–1,870 cc). A step up in punch for touring and Softail models.

• 117 (about 1,917–1,920 cc). Found on many current standard touring models and some CVO bikes.

• 121 (roughly 1,977–1,983 cc). The biggest regular-production engine, exclusive to CVO models.

For a sense of where the standard models fall, take the 2026 Road Glide. It runs the Milwaukee-Eight 117. The 121s are reserved for the premium CVO trim. Regular production reached 121 cubic inches, with variable valve timing introduced in 2023, per Harley's own engine history.

So for the typical rider buying off the showroom floor, the largest displacement they'll get on a standard model is the 121, tucked into the CVO Road Glide ST and other CVO machines.

Key Takeaway: The biggest engine most Harley buyers will ever get their hands on from the regular lineup is the Milwaukee-Eight 121 at roughly 1,977 cc.

The biggest Harley-Davidson engine from the factory: the Screamin' Eagle 131

Now the record gets interesting. If you want the biggest displacement Harley has ever put into a production motorcycle, skip past the 121 and look at the CVO Road Glide RR.

That bike carries the Screamin' Eagle 131, a 131-cubic-inch V-twin that works out to about 2,147–2,153 cc depending on how the rounding shakes out. Harley's own specs on the 2025 CVO Road Glide RR list it at 131 cubic inches, and the moto press agrees it's the most powerful Milwaukee-Eight ever factory-installed in a Harley. Cycle World's first ride of the CVO Road Glide RR called it the largest and most powerful production model the company has ever built, with a claimed 153 horsepower and 150 pound-feet of torque.

That engine is essentially a race-bred piece of hardware dropped into a limited, expensive bagger. It's real, it's factory-installed, and it sets the record for production displacement. It just isn't the kind of bike most people are cross-shopping.

The biggest Harley-Davidson engine ever built: the Screamin' Eagle 135

Here's where the "biggest ever" crown actually lives. Separately from the production lineup, Harley sells crate engines through its Screamin' Eagle performance program. A crate engine is a complete motor sold as a unit for you to install into a compatible bike. It's not fitted to a standard production model line.

The biggest of these is the Screamin' Eagle Milwaukee-Eight 135. Harley's crate engine page states plainly that it delivers the most displacement and most power the company has ever created for a street-compliant crate engine. At 135 cubic inches, it's about 2,212 cc.

The step below it is the Screamin' Eagle 131, at roughly 2,152 cc. Harley's official 131 spec sheet lists a 4.310-inch bore, a 4.50-inch stroke, and a 10.7:1 compression ratio. The 135 spec sheet stretches the stroke to 4.625 inches and bumps compression to 11.7:1.

Neither of these crate motors is what rolls off the assembly line. They're the upgrade path for riders who already own a compatible touring Harley and want more displacement than the factory will give them.

Production vs. crate: the honest distinction

This is the part that trips up a lot of articles on the topic, so it's worth spelling out.

• Biggest regular-production engine across the lineup: Milwaukee-Eight 121.

• Biggest engine ever installed in a production Harley model: Screamin' Eagle 131 in the CVO Road Glide RR.

• Biggest street-compliant engine Harley has ever built: Screamin' Eagle 135, as a crate engine.

None of these answers cancels out the others. If you search "biggest Harley engine," you'll find sources landing on all three, and a quick look at something like SlashGear's breakdown or the rider forum discussions around the 135 crate engine shows real riders talking past each other for exactly this reason.

The cleanest way to keep it straight: the 121 is what you can buy on a CVO model, the 131 is the biggest ever bolted in from the factory, and the 135 is the biggest crate motor Harley has ever offered.

So which one matters for you?

If you're riding an 114 or 117 today, you already have a capable touring motor that moves a big bike without drama. Chasing displacement for its own sake isn't always the smartest money, especially given what CVO trim and crate-engine installs cost.

Where big displacement genuinely pays off is at highway speed and with a loaded bike. More cubic inches mean more low-end torque, which means less hunting for gears and more relaxed cruising. That's the same reason riders load up their baggers with touring gear, wind protection, and yes, the occasional American flag mounted to the luggage rack for a rally or a memorial ride.

If you're interested in seeing how the current 2026 models line up by engine and trim, this Harley 2026 lineup comparison is a helpful starting point. And if you're weighing two of the most popular touring platforms, this Road Glide vs Street Glide breakdown walks through the real differences.

Bottom line

The biggest displacement answer depends on what you mean. For a regular production bike, it's the Milwaukee-Eight 121. For factory-installed production, it's the Screamin' Eagle 131 in the CVO Road Glide RR. For the biggest engine Harley has ever built, it's the Screamin' Eagle 135 crate motor.

Know which one you're asking about, and you'll have the right answer every time.

If you ride a big twin and want it set up to fly the flag safely at speed, riders in the MotorFlagKing community have real-world setups worth looking at. Just confirm fitment for your exact model and year before you order anything.