An English cylinder-head specialist, whose pioneering work on four-valve combustion chambers inspired generations of engineers. At Joe Petrali's suggestion, he was brought in to develop Harley-Davidson's eight-valve racer for 1915. Walter Davidson refused at first, faced with the cost of Ricardo's steamship ticket and his fee, but relented when Petrali reminded him that $25,000 had already been spent on the eight-valve. Ricardo duly worked his magic and the bike was transformed into a race winner. Less successful was the side-valve Model A 21ci (350cc) single that Harry Ricardo had had a hand in and which had a tendency to seize if ridden hard for any length of time.