First introduced in 1926, Percy Riley's 9hp twin-cam four was an outstanding engine design by any standards, various developments powering Rileys until 1957.

Looking to all intents and purposes like a twin-overhead-camshaft design, the Nine's cross-flow cylinder head featured hemispherical combustion chambers and valves inclined at an included angle of 90 degrees.

The twin, gear-driven camshafts were mounted high in the block, operating the valves via short pushrods.

Riley's 9hp engine was one of the most advanced of its day so it can have caused little surprise when the Coventry manufacturer created its first six by, in effect, adding a pair of cylinders to the existing design.

Hailed as 'The Wonder Car Plus Two', the first 14hp, 1.6-litre six-cylinder models duly appeared in 1929, followed by 12hp, 1.5-litre versions in 1933.

Clothed in stylish coachwork by Stanley Riley, the marque's pre-war offerings were among the world's finest small-capacity sporting cars, the 'airline'-bod...