The first V-twin engine used by Harley-Davidson motorcycles was developed based on an earlier engine design. In 1909, Harley-Davidson introduced its first V-twin engine, known as the Model 5-D. This V-twin engine was an evolution of the company's previous single-cylinder engine design.
Harley-Davidson's single-cylinder engines, which were produced starting in 1903, provided the foundation for the development of the V-twin. The early single-cylinder engines were small and had limited power output. As the demand for more power and performance grew, the decision was made to combine two of these single-cylinder engines into a V-shaped configuration.
The V-twin engine design offered several advantages, including increased power, smoother operation, and improved cooling. The Model 5-D V-twin engine featured a 45-degree V-angle and a displacement of 49.5 cubic inches (approximately 811 cc). It became a significant milestone for Harley-Davidson, setting the stage for the iconic V-twin engine architecture that would become synonymous with the brand in the years to come.