It was the most numerous of their single sleeve valve (Burt-McCollum, or Argyll, type) designs, powering many aircraft in the mid-World War II timeframe.

The Hercules powered a number of aircraft types, including Bristol's own Beaufighter heavy fighter design, although it was more commonly used on bombers.

The Hercules also saw use in civilian designs, culminating in the 735 and 737 engines for such as the Handley Page Hastings C1 and C3 and Bristol Freighter.

Shortly after the end of World War I, the Shell company, Asiatic Petroleum, commissioned Harry Ricardo to investigate problems of fuel and engines.

[2] The rationale behind the single sleeve valve design was two-fold: to provide optimum intake and exhaust gas flow in a two-row radial engine, improving its volumetric efficiency and to allow higher compression ratios, thus improving its thermal efficiency.

Also, as combustion chambers of sleeve-valve engines are uncluttered by valves, especially hot exhaust valves, so being comparatively smooth they allow engines to work with lower octane number fuels using the same compression ratio.

Fedden had experimented with sleeve valves in an inverted V-12 as early as 1927 but did not pursue that engine any further.

Reverting to nine cylinder engines, Bristol had developed a sleeve valve engine that would actually work by 1934, introducing their first sleeve-valve designs in the 750 horsepower (560 kilowatts) class Perseus and the 500 hp (370 kW) class Aquila that they intended to supply throughout the 1930s.

Aircraft development in the era was so rapid that both engines quickly ended up at the low-power end of the military market and, in order to deliver larger engines, Bristol developed 14-cylinder versions of both.

At that time, the tolerances were simply not sufficiently accurate to ensure the mass production of reliable engines.

Fedden drove his teams mercilessly, at both Bristol and its suppliers, and thousands of combinations of alloys and methods were tried before a process was discovered which used centrifugal casting to make the sleeves perfectly round.

[2] In 1937 Bristol acquired a Northrop Model 8A-1, the export version of the A-17 attack bomber, and modified it as a testbed for the first Hercules engines.

Hercules I (1936) – 1,150 hp (860 kW), single-speed supercharger, run on 87 octane fuel.

[5] Hercules II (1938) – 1,375 hp (1,025 kW), single-speed supercharger, run on 87 octane fuel.

[5] Hercules III (1939) – 1,400 hp (1,000 kW), two-speed supercharger, run on either 87 or 100 octane fuel.

[6] Hercules IV (1939) – 1,380 hp (1,030 kW), single-speed supercharger, run on 87 octane fuel.

[6] Hercules VI (1941) – 1,615 hp (1,204 kW), two-speed supercharger, run on either 87 or 100 octane fuel.

[8] Hercules XIV (1942) – 1,500 hp (1,100 kW), developed for the civil market and used by BOAC, run on 100 octane fuel.

[8] Hercules XVI (1942) – 1,615 hp (1,204 kW), two-speed supercharger, run on either 87 or 100 octane fuel.

[8] Hercules XVII (1943) – 1,615 hp (1,204 kW), two-speed supercharger locked in 'M' gear.

The Hercules 134 was a development with modified mounting ring and exhaust pipes for a rear manifold.