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Weslake was a cylinder head specialist who had been instrumental in modifying the side valve standard engine used in the first SS (Swallow Sidecars - later to become Jaguar) sports car. He also worked on the larger SS engine: "The 2½-litre car has an o.h.v. power-unit with Weslake combustion chambers, and gives over 40 b.h.p. per litre."  He also designed the cylinder head for the overhead valve version of the Austin 'A' series engine that was used in the Morris 1000 and the Mini and received royalties on each of these engines manufactured. He was involved in the design of every Jaguar engine up to and including the V12 of the early 1970s. Weslake was also involved in the development of the Rolls-Royce Merlin engine.
In 1966 Dan Gurney commissioned Weslake Engineering to build an Aubrey Woods designed 3.0-litre V12 Formula One engine for his Eagle Mk1. Their efforts produced a V12 that was smooth and powerful. At Monza, an insight into the future of engine design was seen for the first time. The engine had four valves per cylinder at a narrow included angle (thirty degrees) that allowed a single cover to enclose both the close-spaced camshafts on each bank. The sixty-degree-vee layout had a larger bore than stroke (72.8 × 60 mm). Gurney won the Race of Champions at Brands Hatch, a non-championship event, and the 1967 Belgian Grand Prix with the Eagle-Weslake V12 engine.
At Monza in 1966, 364 bhp (271 kW) was available. This increased to 390 bhp (290 kW) during the winter. At the 1967 Brands Hatch Race of Champions, Dan Gurney's engine gave 413 bhp (308 kW) and Richie Ginther's engine gave 409 bhp (305 kW). On test, up to 422 bhp (315 kW) had been achieved. At Monaco, Gurney had 411 bhp (306 kW), Ginther 417 bhp (311 kW). Later in the 1967 season quotes of 416 bhp (310 kW) were made. (These are figures from Motoring News.) The engines peaked at around 10,000 rpm. A figure of 442 bhp (330 kW) was mentioned at the start of 1968, but after money ran out, a test made at the B.R.M. factory recorded only 378 bhp (this may have been a 'tired' engine). Harry Weslake had an eventual goal of 500 bhp (370 kW) @ 12,000 rpm. Later Ford sponsored (75.0 × 56.25 mm) versions in 1972 were quoted at 465 bhp (347 kW) @ 10,500 rpm.
In addition to three Jaguar Le Mans victories in the 1950s, Harry Weslake and his company provided the Gurney-Weslake cylinder heads for the engines that powered the Gulf-Wyer Ford GT40 Mk.I to two consecutive wins at Le Mans, in 1968 and 1969.
During the 1970s, Weslake manufactured the Cologne RS2600 engine that Ford fitted to the Capri. This also included the special Weslake aluminium heads used for Ford's touring car challenge. The Weslake Ford Capri went on to win its class at Le Mans in 1972 and won all but one round of the European Touring car championship outright in the same year.
Weslake Engineering went on to design a series of successful motorcycle engines during the 1970s that were also used in early shifter karts.
Peter Collins of Belle Vue Aces and England won the 1976 Speedway World Final on a Weslake bike. Bruce Penhall rode a Weslake speedway motorcycle to many successes in the early 1980s, including two World Individual Speedway Championships.  Harry Weslake was awarded the Segrave Medal in 1976 for his part in developing the four-stroke speedway engine.
Harry Weslake died in 1978.
- Karl Ludvigsen, Gurney's Eagles: The Fascinating Story of the AAR Racing Cars, Page 31.
- Motor Sport, November 1935, Page 35.
- "Harry Weslake", Speedway Star, 9 September 1978, p. 25
- American Motorcyclist, December 1992, Page 55.