High-powered 250cc V4 machine
To compete against the Honda V-6 machine introduced in 1964, Yamaha developed the RD05 with a 2-stroke V-4 engine as its new race machine to replace the RD56. The new V-4 engine used the proven rotary disc valve, and to achieve sufficient V bank angle it was designed with the concept of two RA97 air-cooled 2-cylinder engines stacked one on top of the other. The RD05 debuted in the 1965 Italian GP. For the next round it was outfitted with liquid cooling but its only GP victory would come at the Japan GP of the 1966 season. For the 1967 season it was completely re-engineered as the RD05A with every possible effort made to reduce weight and size. Although RD05A fell short of winning the title that year, it returned to win both the Manufacturer and Rider titles in 1968.