The most well-known of Ariel’s engines, the Square Four, was introduced in 1931.

   The Square Four offered four air-cooled cylinders set in a square formation.  Ed Turner conceived the Square Four engine in 1928.  At the time, he was looking for work and showing drawings of his engine design to various motorcycle manufacturers.

   The engine was essentially a pair of ‘across frame’ OHC parallel twin cylinders joined by their geared central flywheels, with a one-piece four-cylinder block (or Monobloc) and single head.
 
   The idea for the engine was rejected by BSA, but adopted by Ariel.  Thus it became the Ariel Square Four.

​​Block Mfg.: Ariel   
Cylinders: 4
CID: 30 (500 cc)
Bore: 65mm
Stroke: 75mm
Head Mfg.: Ariel 
Material: Aluminum
Cam: Overhead, chain driven