The Mercury was developed by the Bristol Aeroplane Company in 1925 as their Bristol Jupiter was reaching the end of its lifespan. With the widespread introduction of superchargers to the aviation industry in order to improve altitude performance, designer Roy Fedden felt it was reasonable to use a small amount of boost at all times in order to improve performance of an otherwise smaller engine.
Instead of designing an entirely new block, the existing Jupiter parts were re-used with the stroke reduced by one inch (25 mm). The now-smaller capacity engine was then boosted back to Jupiter power levels, while running at higher rpm and thus requiring a reduction gear for the propeller.
The Mercury is a 9-cylinder, single row radial engine with conventional ‘poppet’ valves, unlike later Bristol sleeve-valve engines. This engine has four valves per cylinder – an unusual feature for a radial engine.
A total of 32,000 Mercury engines were produced by Bristol and other companies between 1939 and 1945. They were used in the Bristol Blenheim/Bolingbroke, Westland Lysander, Supermarine Sea Otter, Gloster Gladiator and Miles Martinet.