Flat-twin engine
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A flat-twin engine is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the boxer-twin engine, where both pi
Douglas 80 Plus motorcycle engine (circa 1950)
Citroën 2CV engine (viewed from rear)
1912 Douglas N3 engine
1942 Harley-Davidson XA engine
Harley-Davidson Model W
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The Harley-Davidson Model W, also known as the Sport Twin, is a motorcycle made by Harley-Davidson from 1919 to 1923. Unusually for a Harley-Davidson motorcycle, the Model W had a flat-twin engine and
1921 Harley-Davidson Model W
W. S. Harley's patented compression release system. Note the screw caps in the cylinder heads giving access to the valves.
1922 Harley-Davidson WJ
Beam-framed motorcycle with Model W drivetrain, patented by W. S. Harley and A. Ziska Jr.