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A straight-twin engine, also known as an inline-twin, vertical-twin, or parallel-twin is a two-cylinder piston engine where two cylinders are arranged in a line along a common crankshaft.

Straight-twin engines are primarily used in motorcycles; other uses include automobiles, marine vessels, snowmobiles, jetskis, all-terrain vehicles, tractors and ultralight aircraft.

Various different crankshaft configurations have been used for straight-twin engines, with the most common being 360 degrees, 180 degrees and 270 degrees.

Other usesIn the early 20th century, gaff-rigged British fishing boats such as Morecambe Bay PrawnersLancashire Nobbys would sometimes retrofit an inboard engine, such as the Lister or the Kelvin E2 3.0 litre petrol-paraffin engine.

From the 1950s, manufacturers of outboard motors had settled on the use of the basic inline engine design, cylinders stacked on top of each other with the crankshaft driving the propellor shaft. The Suzuki 15 outbound motor was introduced in 1989.[54]

Other uses include tractors (such as various John Deere models until 1960),[55] snowmobiles,[56][57] jet skis,[58][59] and all-terrain vehicles.[60][61][62] Design varations include two-stroke, four-stroke, petrol, diesel,[63] air-cooling, water-cooling, natural aspiration and turbocharging.[64]

Ultralight aircraft, single seat gyro-copters and small homebuilt aircraft have also used straight-twin engines,[65] often using engines originally designed for snowmobiles such as the Hirth 2704 and Cuyuna 430-D.[56][66][67] Purpose-built engines for ultralight aircraft include the Rotax 503 and Rotax 582.[68] Straight-twin engines are sometimes also used in large scale

Ultralight aircraft, single seat gyro-copters and small homebuilt aircraft have also used straight-twin engines,[65] often using engines originally designed for snowmobiles such as the Hirth 2704 and Cuyuna 430-D.[56][66][67] Purpose-built engines for ultralight aircraft include the Rotax 503 and Rotax 582.[68] Straight-twin engines are sometimes also used in large scale radio-controlled aircraft.[69][70]