thumb|right|1964 [[Sanglas#History|Sanglas Rovena motorcycle engine (built by Hispano Villiers)]]
thumb|1997 [[Suzuki GS500 motorcycle engine]]
A straight-twin engine (also known as an inline-twin, vertical-twin, inline-2, or parallel-twin) is a two-cylinder piston engine whose 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, jet skis, 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.
Terminology
The straight-twin layout is also referred to as "parallel-twin", "vertical-twin" and "inline-twin". This architecture is proving popular among manufacturers, which are upgrading models that were previously equipped with other engine types, such as the 2016 Honda Africa Twin (formerly a V-twin), 2023 Honda Hornet (formerly an inline-4) or 2023 V-Strom 800 (the older design being equipped with a V-twin).
Main bearings
Each cylinder in a straight-twin engine has a separate crankpin, unlike V-twin engines which can use a common crankpin for both connecting rods. Most vintage British straight-twin motorcycle engines (such as Triumph, BSA, Norton and Royal Enfield) had two main bearings.
The 1938 Triumph Speed Twin was a successful straight-twin motorcycle which also led to straight-twin engines becoming more widely used by other brands. The engine was designed by Edward Turner and Val Page, and was initially used in the 1933 Triumph 6/1 sidecar hauler (which won the International Six Days Trial silver medal and the 1933 Maudes Trophy). During the development of the engine, it was found that a 360 degree crank angle was better suited to the use of a single carburettor than a 180 degree crank angle.
Following the trend created by the Triumph Speed Twin, the most common design used by British motorcycle manufacturers until the mid-1970s was a four-stroke straight-twin engine with a 360 degree crankshaft.
The transverse-engine straight-twin design has been largely replaced by V-twin engines; however, the straight-twin design has the advantage of easier packaging of ancillaries (such as the air-filter, carburettor and ignition components), which also improves access to ancillaries for maintenance/repairs. A straight-twin engine using a 270 degree crankshaft can have a similar sound and feel to a V-twin engine with an uneven firing order.
Longitudinal-engined models
thumb|1951 [[Sunbeam S7 and S8|Sunbeam S8 ]]
Longitudinal engine straight-twin motorcycles are less common; however, examples include the 1930–1938 Dresch Monobloc and the 1949–1956 Sunbeam S7 and S8. Another early straight-twin engined car was the 1898 Decauville Voiturelle, which used a pair of cylinders taken from a de Dion model mounted fore and aft and positioned below the seat. 1988 Dacia Lăstun, 1980 Daihatsu Cuore, and the 2008 Tata Nano.
Usage in marine vessels
thumb|Easthope marine engine, circa 1960
Straight-twin engines have been often used as inboard motors, outboard motors and jet pump motors.