Square engine

The square engine is an engine configuration used on some 4-cylinder motorcycles like the Ariel Square Four. It's easiest to think of a square-four engine as a pair of straight-twin (AKA parallel-twin) engines with their crankshafts mated together using gears to give a common output.

This design was revived as a two-stroke version on some racing Suzukis, and their subsequent road-going version the Suzuki RG500. Although some racing success was achieved, the road bikes didn't sell in great numbers, and the design was phased out in favour of in-line, four-stroke designs, as at the time two stroke engines were quickly being superseded by more economical and reliable four strokes.

The engine is only referred to as "square" when four cylinders are utilised, otherwise the engine is a U engine.

An engine can also be referred to as "square" when the bore and stroke are exactly equal.