In 1903, William Harley and brothers Arthur and Walter Davidson pieced together the first H-D motorcycle. They built three single-cylinder engine motorcycles that year. Their first production model V-twin motor, displacing 50 cubic inches, was released in 1909; a 61-cubic-inch version was added in 1912.
Early H-D single-cylinder motors had overhead intake valves that were opened by vacuum rather than mechanical cam action. The first V-twin 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.
The V-twin, which had grown from 50 cubic inches to 61 for 1912, was joined by a 74-cubic-inch version in 1921 -- the first of the famed "Seventy-fours." Improvements were made to the V-twin's original IOE design over the years, but by the late 1920s they still had exposed valve trains that were messy to run, difficult to maintain, and highly susceptible to wear.
In 1929, the 45-cubic-inch V-twin engine (later to be known as the “flathead”) was introduced on the D model. That engine proved to be so reliable that variations of it were available on H-D motorcycles as late as 1973.