Comet designed the 7-cylinder radial series to take advantage of the boom in private aviation in the late 1920s; intending to replace the large number of relatively unreliable wartime surplus engines that flooded the market in the early 1920s.

Oil pumps and magnetos are driven by the camshaft drives as well as the Heywood air starter and tachometer.

The two-piece crankshaft, with counter-weights, is carefully forged from heat-treated chrome-vanadium alloy steel and drilled for lightness and lubricating oil flow.

The rear of the crankshaft carries an induction rotor to ensure even mixture distribution to the cylinders from the Stromberg carburettor.

The two-piece crankcase is split in the plane of the cylinder centres, with the forward half supporting the fwd crank /prop-shaft bearing and the rear half housing the rear bearing, induction rotor, cam drives and accessories.