1903-1918 Single Cylinder
The first successful motorcycle engine, created by William Harley and Arthur Davidson, was a 24.74 cubic inch F-Head single-cylinder, known as the original single-cylinder. Developed between 1902 and 1903 in collaboration with Milwaukee inventor Ole Evinrude, its output was approximately 3 hp. Constantly improving, Harley-Davidson offered a single-cylinder F-Head with displacement up to 35 cubic inches until the 1918 model year.
1909 V-Twin
With its cylinders arranged at a 45-degree angle, this early V-twin engine established a key design element that endures to this day. This engine had a displacement of 49.5 cubic inches and generated about 7 hp. Unfortunately, it proved unreliable at times and was only offered for one year.
1911 - 1929 F-Head
Harley's second attempt at a V-twin engine was a success. With an improved valvetrain, sealed ignition, and a displacement of 61 cubic inches, the new engine was reliable and powerful. The displacement was later increased to 74 cubic inches.
1929 – 1973 Flathead
The side-valve Flathead engine would prove to be a durable workhorse, and was the engine that powered Harley-Davidson military models during World War II. Initially offered as the D model in 1929 with a displacement of 45 cubic inches, a 74 cubic inch V model was offered in 1930 as the powerful new engine in the Harley line. In 1937, the U and UL models were powered by 74 cubic inch Flatheads, while the UH and ULH received an 80 cubic inch version. The Flathead engine powered the Harley-Davidson Servi-Car utility trike until 1973. Harley fans, inspired by its flat, finned cylinder heads, gave this engine the nickname Flathead.
1936 – 1947 Knucklehead
Harley-Davidson's first overhead valve engine was nicknamed the Knucklehead by Harley aficionados, who considered its rocker covers and pushrod tubes to resemble a closed fist. The first E-model engine retained Harley's signature 45-degree cylinder angle and 61 cubic inches of displacement, with a 74 cubic inch version later available. The Knucklehead engine established the basic architecture for all future air-cooled Harley-Davidson V-Twin engines.
1948 – 1965 Panhead
Updated with aluminum cylinder heads, hydraulic tappets, improved lubrication, and rod bearings, this second-generation Big Twin was 8 pounds lighter than the Knucklehead. Displacement remained at 61 and 74 cubic inches. Once again, Harley fans nicknamed the new engine the Panhead, due to the new stamped rocker covers that, to some, resembled cake pans.
1952 – 1956 K-Model
The sport version of the Flathead engine had a displacement of 742 cc and 30 hp. In 1954, Harley-Davidson increased engine power to 38 hp and a top speed of 161 km/h (99 mph). A total of 1,250 units were built, serving as the basis for the KK series racing engines and as precursors to the Sportster engines.
1957 – 1985 Ironhead Sportster
Designed for the new XL Sportster model, this 55 cubic-inch V-twin engine featured 45-degree cylinders, overhead valves, aluminum pistons, and a hemispherical combustion chamber. The Ironhead Sportster was a "unit" engine, with the transmission housed in the same case as the engine and incorporating a foot shifter. Its iron cylinder heads earned it the nickname "Ironhead."
1966 – 1984 Shovelhead
Introduced with the first Electra Glide model, this Big Twin engine update included redesigned aluminum cylinder heads, beefier valves and pistons, and a 12-volt electrical system. Available in 74 and 80 cubic inch displacements, the "Shovelhead" nickname comes from the shape of the new rocker covers, which resemble "shovels," or resemble the shape of an old-fashioned coal shovel.
1984 – 1999 V2 Evolution
All-new and the first Harley engine created using computer-aided design, the 80-cubic-inch "Evo" retained the classic 45-degree layout and overhead valves with pushrods, but featured aluminum cylinders and heads, a new ignition system, and a combustion chamber that generated power using lower-grade unleaded gasoline while improving fuel economy. Advances in design and manufacturing made this new engine airtight. The 1995 Electra Glide Ultra Classic model offered an Evolution engine with electronic fuel injection, a first for Harley-Davidson. The Evolution engine is known for its ruggedness and reliability.
1986-2022 Sportster Evolution
This new engine for Sportster models incorporated four overhead camshafts and many design elements derived from the Evolution Big Twin V2 engine. Initially offered with an 1100 cc displacement, later versions of the Sportster Evolution engines were available in either 883 cc or 1200 cc versions.
1999 – 2017 Twin Cam
Named for its dual overhead cam design, this new V-Twin had a displacement of 88 cubic inches and shared only 18 parts with the Evolution engine it replaced. The Twin Cam's displacement would eventually reach 110 cubic inches. New cylinder heads, valves, and intake and exhaust ports optimized emissions, fuel economy, and performance. B-series Twin Cam engines featured a balancer countershaft to reduce vibration in rigid-mount Softail models.
2002 – 2017 Revolution
Designed for the new VRSCA V-Rod motorcycles, the 69 cubic inch (1131 cc) Revolution engine represented a radical departure from previous Harley-Davidson engine design. Developed in collaboration with Porsche, the Revolution V-Twin's two cylinders were spaced 60 degrees apart and featured dual overhead camshafts and four-valve cylinder heads. The liquid-cooled, internally counterbalanced, fuel-injected Revolution V-Twin engine developed 115 hp at 8200 rpm, a new benchmark for a Harley-Davidson production engine.
2015 - 2020 Revolution X
The Harley-Davidson XG's Revolution X engine is a modern, liquid-cooled V-Twin designed for agile performance in urban environments. It was offered in 500cc and 750cc displacements, with four valves per cylinder and a single overhead camshaft (SOHC) for smooth, controlled power delivery. This engine represented a new generation of engines for the brand, focused on a younger, more global audience.
2017 - Current Milwaukee Eight V-Twin
Featuring an all-new design, the Milwaukee-Eight or M8 engine features a return to a single overhead camshaft, but with four-valve cylinder heads (two exhaust and two intake per cylinder), giving rise to its name. All versions are counterbalanced to reduce vibration, and some feature Twin-Cooled liquid-cooled cylinder heads. Initially available in 107 and 114 cubic inches, displacement has increased to 121 cubic inches in regular production, with the introduction of Variable Valve Timing in 2023. With its traditional 45-degree cylinder angle, the Milwaukee-Eight V-Twin engine retains the classic look, sound, and feel of a Harley-Davidson V-Twin engine.
2017 - Revolution Max V-Twin
Representing a new generation of Harley-Davidson power, the Revolution Max engine family debuted on the Pan America and Sportster S models. This 60-degree, liquid-cooled V-Twin was designed to function as a reinforced element of the motorcycle's frame, reducing its overall weight. The Revolution Max V-Twin engine is optimized for light weight and a broad powerband that generates a burst of power at high RPMs. It features dual overhead camshafts, four-valve cylinder heads, variable valve timing, dual downdraft throttle bodies, and is counterbalanced to reduce vibration. The engine and six-speed transmission share a common gearbox. The Revolution Max is available in 1250cc and 975cc displacements.
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