 Built largely from cast Aluminium alloys the 12Aa was a geared engine with four Zenith carburetors supplying mixture to the lower intake valves.Four camshafts at the corners of the crankcase operated two intake valves and two exhaust valves per cylinder, also driving oil pumps and other accessories.The four magnetos were mounted on transverse shafts driven from the crankshaft at the front of the engine.