The Square Four, introduced in 500cc form during 1931 was the work of Edward Turner. Conceived as a luxury tourer it utilised a chain driven over camshaft and four cranks with the gearbox being driven by the rear left hand example. The new model was joined by a 601cc variant the following year which eventually replaced the smaller machine. Unfortunately for Ariel, the introduction of complex, luxury model into an already crowded market place at the height of a recession resulted in limited sales, many admired the machine but those that could afford a luxury mount tended to opt for the known qualities of one of the large capacity vee twins, consequently examples of either over head cam model are rare. In an attempt to broaden the appeal of their range leader Ariel replaced the cammy 600cc Square Four with an overhead valve design displacing 997cc during 1936. The redesigned engine retained the geared twin crankshafts first seen in the overhead cam engine, although the gears had moved to the left hand side of the engine, but little else, other than the name, remained unchanged from the cammy Square Four.
This example, dating from 1937 was restored during 1990 and is described by the vendor as being "an excellent restoration of a pre war Square Four". Attractively presented with black paintwork it is offered with a Swansea V5C and is described as running "perfectly" by the vendor.