Prior to 1969 the American Motorcyclist Association (AMA) rules for the AMA Grand National Championship were deliberately structured to favor side-valve engines rather than overhead-valve engines. The result of this was to favor American made bikes such as those from Harley-Davidson with their side-valve engines, and to disadvantage the overseas competition which was mainly from British motorcycles especially Triumph, BSA, and Norton. The AMA rules prior to 1969 allowed side-valve engines of up to 750cc capacity but OHV engines were limited to 500cc. With their 50% engine size advantage the odds were stacked in the favor of the side-valve bikes.
The British motorcycle manufacturers put up with that for a while but by the late sixties their market in the USA for bikes around 500cc was shrinking and customer demand for bikes in the 650cc and 750cc classes was increasing. So the British worked to get the AMA to level the playing field and in 1969 the sought after rule change was enacted so that both side-valve and overhead-valve engines of up to 750cc were allowed in the AMA Grand National Championship. The effect of this on Harley-Davidson was to precipitate the need for a new OHV-engined racing bike that could successfully compete against the Brits and keep the Stars and Stripes at the forefront of AMA competition.