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.