The flat-plane crank (sometimes flatplane) is a type of crankshaft for use in internal combustion engines that has a 180-degree angle between crank throws.^[1]

Flat-plane cranks are used in V-configuration engines, generally with eight cylinders.^[1] Cadillac introduced a V8 flat-plane crank engine in 1923.^[2][3]

Inline-four cylinder engines almost all use flat-plane cranks, and thus are not usually identified as such. However, there are a few exceptions with crossplane cranks.

The flat-plane crankshaft was used in the World War II Sherman Tank in the Ford GAA engine, an all-aluminium 32-valve DOHC 60-degree liquid-cooled V8. However, the flat-plane design is no longer widely used in most mass production V engines as it is more prone to vibration and is inherently much louder than a crossplane crankshaft. However, due to its simpler construction requiring less counterweight, it is inherently lighter with a higher rev limit. For this reason, it remains useful in sports and racing cars.^[1] Flat-plane crankshafts are generally associated with European sports cars such as Ferrari and Lotus V8 engines, and cross-plane cranks with American manufacturers. There are some exceptions such as the Ferrari-designed crossplane crank V8 of the Lancia Thema 8.32 and the flat-plane crank 2014–2023 Ford Mustang GT350.^[4] The 2023 model year Chevrolet Corvette Z06 has the largest flat-plane V8 ever seen in production cars at 5.5 litres.^[5]

The way in which a flat-plane works within a V8 engine is more like two in-line 4-cylinder engines mated together,^[1] with the firing order of each order being in a Right-Left-Right-Left-Right-Left-Right-Left pattern.^[6] Being in this in-line configuration allows for the engine to rev much faster, making it more suitable for racing engines.

• Crossplane