When one thinks of Harley-Davidson engines, thundering V-twins is what usually springs to mind. Though different designs in a number of different sizes have been produced over the years, the largest versions have always been referred to as Big Twins (though the same term is also used for the bikes that carried those motors). There have been six distinct Big Twin design generations, and the company's history is often segmented in accordance with those six generations -- as we have in this article.
One more note. While the general rule is that engines are gas-powered and motors are electric-powered, Harley-Davidson (as well as most motorcycle historians) typically refer to the powerplants as motors -- probably because the vehicles have always been called motorcycles, not enginecycles -- and we will follow suit.
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1909-1929 Intake Over Exhaust (IOE): Harley's early single-cylinder motors had overhead intake valves that were opened by vacuum rather than mechanical cam action. The first V-twin introduced in 1909 likewise had vacuum-operated intake valves-which didn't work particularly well. When it was re-introduced in 1911, the V-twin had normal cam-actuated intake valves. Exhaust valves were mounted in the block (side valve) in all cases. Early V-twins displaced 50 cubic inches; a 61-cubic-inch version was added in 1912.
1930-1935 Flathead (side-valve): Big Twin flatheads led the line for only six years before being overshadowed by Harley's first overhead-valve V-twin. However, flathead Big Twins remained available through 1948, selling alongside their more modern siblings. Original Big Twin flatheads displaced 74 cubic inches; an 80-inch version was added late in 1935. Though flatheads were theoretically less efficient than the former IOE designs, they had evolved to the point where they were not only more powerful, but also easier to work on-a major advantage in the days when motors required more maintenance and weren't as reliable as they are today.
1936-1947 Knucklehead: Harley had offered overhead-valve singles in the 1920s, but the Knucklehead was the company's first overhead-valve V-twin. The original Knucklehead displaced 61 cubic inches, but a 74-inch version was added for 1941. Though smaller in displacement than concurrent flathead models, the Knucklehead's overhead-valve configuration gave it more power. Introduced on the Knucklehead was a modern recirculating oiling system. Incidently, "Knucklehead" was not Harley's name for the motor (the factory called it the "61 OHV"). Riders gave it the "Knucklehead" nickname because its valve covers looked like fists with two knuckles sticking out.
1948-1965 Panhead: The Panhead motor was given new aluminum heads that ran cooler than the former cast-iron ones and helped produce more power. It also introduced hydraulic valve lifters, which minimized the tedious task of adjusting valves and also allowed the motor to run more quietly. Like the Knucklehead before it, riders coined the nickname "Panhead" in reference to the motor's valve covers, which now looked like upside-down roasting pans. Initially offered in 61- and 74-cubic-inch sizes, the 61 was dropped after 1952. In its final year, the Panhead was fitted with an electric starter -- a real convenience, as Big Twins had become notoriously hard to kick over.
1966-1983 Shovelhead: Redesigned heads gave the Big Twin a bit more power, and the valve covers topping them now looked like upside-down shovel scoops-hence the "Shovelhead" nickname that was once again bestowed by riders. Initially sized at 74 cubic inches, an 80-inch version bowed for 1978 and eventually became the sole offering. Though the new Evolution motor arrived in 1984, some Big Twin models continued to carry a Shovelhead for that year.
1984-present Evolution V2: Harley's Evolution V-twin addressed many of the complaints leveled against the Shovelheads, as it was smoother, quieter, more powerful, and more reliable. Though some riders early on began referring to the new motor as the "Blockhead" due to its smooth, block-shaped valve covers, the name never stuck and this latest generation of the 80-cubic-inch Big Twin is usually referred to as the "Evo."
Keep reading to learn about Harley's smaller twin engines.
To learn more about Harley-Davidson and other classic motorcycles, see:
Harley-Davidson Engines: Little Twins
Since 1929, Harley-Davidson engines have also been offered as smaller V-twin motors. Sometimes these smaller models are very difficult to distinguish from their larger brothers, but one rule has remained constant: Big Twins have always carried their final-drive chains (or belts) on the left side of the bike, while the smaller models have always had them on the right. Also, since 1952, the smaller models have had the motor and transmission joined together in one case (called unit construction), whereas Big Twins have always driven through a separate transmission.
1929-1952 Forty-five Flathead (side valve): Harley-Davidson's first flathead V-twin was a small 45-cubic-inch motor that preceded its flathead Big Twin brother by a year. It proved to be an extremely reliable unit, and enjoyed the longest life span of any motor in Harley history: Not only was it used in motorcycles for over 20 years, it powered the three-wheel Servi-Car from its introduction for 1933 through its final edition in 1973! It also led an exciting life, being the powerplant of choice for the motorcycles that served the allies in World War II, as well as for racing machines that racked up a long and enviable winning record.
1952-1956 K-Series Flathead: Originally sized at the same 45 cubic inches as its predecessor, the K grew to 55 cubic inches for 1954. It retained a flathead configuration but became the first Harley V-twin to be mated into one unit with its transmission. Like the original Forty-five, it proved to be disarmingly potent in competition, powering winning racers well into the 1960s.
1957-1985 Sportster: The K-Series motor adopted overhead valves in 1957 to become the famed Sportster. Sized at the same 55 cubic inches as the later K-Series motors, it was referred to by its equivalent displacement in cubic centimeters (883-cc) because that's how the motors of its primary competitors were defined. Early Sportsters were among the quickest motorcycles of their day, being the uncontested "King of the Drags" until the late-1960s. Displacement grew to 1000-cc for 1972, but even that wasn't enough to put the Sportster back on top in the stoplight grand prix.
1986-present Evolution Sportster: After nearly 30 years, the Sportster's motor finally received a freshening for 1986, taking on the look -- and name -- of its bigger Evolution brother. For the first time, it was offered in two sizes: the original 883-cc and a new 1,100-cc version. The latter would grow to 1,200-cc in 1988, which equates to 74 cubic inches -- the same displacement as the largest of the pre-1978 overhead-valve Big Twins.