Harry Weslake was born in Exeter in 1897. After serving with the Royal Flying Corps in World War I, he set up a workshop and began marketing the Wex carburettor design. His growing reputation for gas flow analysis in combustion chambers led to consultancy work for Sunbeam's competition team at Brooklands, where he developed an air flow meter that became the foundation of his technical approach. W. O. Bentley engaged Weslake to tune racing engine cylinder heads, work that contributed to the first Bentley Le Mans victory. After the collapse of his Wex Carburettors company in 1926, Weslake moved to Automotive Engineering before establishing his own business in 1935.
During the 1930s and World War II, Weslake contributed to a remarkable range of projects: development work on the Rolls-Royce Merlin engine for aircraft and tank use, collaboration on the Jaguar XK engine design alongside William Heynes, Walter Hassan, and Claude Baily, and consultancy on the Jowett Javelin flat-four. He also designed the cylinder head for the overhead-valve version of the BMC A-series engine used in the Morris Minor, the Mini, and numerous other BMC products, receiving royalties on each unit manufactured.
Weslake's involvement in Formula One began through the Vanwall team. During 1956, working alongside Leo Kuzmicki, Colin Chapman, and Frank Costin, Weslake helped develop the car that Stirling Moss drove to a win in a non-championship Formula One race, persuading Moss to join the team for 1957. The Vanwall went on to win the inaugural constructors' championship title in 1958. Weslake subsequently acted as a consultant to Coventry Climax, advising on port design for their FPF engine used by Cooper and Lotus.
In 1966 Dan Gurney commissioned Weslake to build an Aubrey Woods-designed 3.0-litre V12 Formula One engine for his Eagle Mk1 constructor effort. The engine had originally been conceived as a BRM proposal but lost out internally to BRM's own H16 design. The resulting Weslake V12 was a technically notable design: a 60-degree V configuration with a bore and stroke of 72.8 x 60 mm, featuring four valves per cylinder at a narrow 30-degree included angle with a single cover enclosing both close-spaced camshafts on each bank — a layout that anticipated later standard Formula One engine architecture.
Initial output at Monza in 1966 was 364 bhp, increasing to around 390-413 bhp during 1967. Gurney won the 1967 Race of Champions at Brands Hatch with the Eagle-Weslake V12, a non-championship event, and then the 1967 Belgian Grand Prix at Spa-Francorchamps — the only world championship Formula One victory for the Eagle constructor, and for the Weslake engine. Financial difficulties ultimately halted further development, and after money ran out a test measured the engine at only 378 bhp from a unit likely worn by accumulated mileage.
Harry Weslake had set a long-term target of 500 bhp at 12,000 rpm. Later Ford-sponsored versions produced in 1972 were quoted at 465 bhp at 10,500 rpm, but the program never achieved the sustained competitive funding needed to challenge the dominant Cosworth DFV.
Weslake supplied the Gurney-Weslake cylinder heads for the Ford GT40 Mk.I engines that powered John Wyer's Gulf Racing team to consecutive 24 Hours of Le Mans victories in 1968 and 1969. In 1970, Harry Weslake agreed to develop a V12 for Ford and JWA for sports car racing, with a 3-litre unit of 75 x 56.5 mm dimensions designed to withstand 24-hour races and allow future bore increases. Development was eventually abandoned following poor test results and the withdrawal of Ford's racing budget after the 1973 oil crisis.
During the 1970s Weslake designed motorcycle engines that found success in speedway racing. Peter Collins won the 1976 Speedway World Final on a Weslake-engined bike, and Weslake's last project before his death in 1978 was developing a 992 cc air-cooled V-twin for Lord Hesketh. Weslake also received the Segrave Medal in 1976 for the development of the four-stroke speedway engine. Harry Weslake died in 1978 while attending the World Speedway Championship at Wembley. The Goodwood Revival Festival has since 2017 honoured his legacy with the Weslake Cup for Spridget racing.