IN DETAIL:
The premier AMA road racing class in the 1960s was production based with a rule that gave side-valve engines a one-third capacity advantage over overhead valve engines!
Most European factories had not used side-valves in race engines since the First World War. The reason for the rule was the influence KR side-valve manufacturer Harley-Davidson had on the AMA board. However, the huge sales potential in the USA market meant companies like Triumph needed the publicity of racing despite the capacity handicap.
