Horizontally Opposed Motorcycle engines from France, Britain, Germany, Switzerland...
The HO flat twin engine was designed and patented by Karl Benz in 1896, who named it the Kontra engine. The flat twin, or boxer, has both conrods on the same big-end journal. A flat four has two big-end journals and probably the best known example is the air-cooled Volkswagen. The design is used in a great number of light aircraft, beginning in 1909 and continuing today with Lycoming, Rotax and others.
Motorcycles which have used such a design include:
Fée (1905) & Fairy
Bradshaw - for a list of marques which used their engines see Bradshaw Oil-Cooled Engines
Victoria. From 1923 they built 497cc and 597cc HO twins (KR2, KR3, KR6 etc.), some of which were supercharged for racing. The KR2 was belt-driven, while the KR3 Sport had a three-speed gearbox and chain drive.
Prior to this they had used BMW engines in their KR1, but built their own engines after BMW began motorcycle production.
Granville Bradshaw, ABC, and later Gnome & Rhone
BFG (Citroen GS)
Honda Goldwing fours and sixes
Riedel, who built the Imme, designed a flat twin for jet aircraft during WWII. It was built by Victoria.
Dneipr of Ukraine adapted a German design and produced large quantities, particularly for sidecar use.
Sources: Wikipedia, moto-collection.org et al.
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