The III is powered by an aluminium-alloy V12 engine of 447in³ (7.32L), having a bore of 3.25 inches (82.5 mm) and a stroke of 4.5 inches (114.3 mm). It is a pushrod engine with overhead valves operated by a single camshaft in the valley between the cylinder banks. Early cars had hydraulic tappets or, rather, a unique system of eccentric bushings in each individual rocker that was actuated by a small hydraulic piston; the eccentric bushing ensuring zero valve-lash at the rocker/valve interface. This system was changed to solid adjustable tappets in 1938. The Phantom III is unusual for its twin ignition systems, with two distributors, two coils and 24 spark plugs. Petrol is provided by a twin SU electric pump. Wire wheels are fitted as standard, but many cars carry Ace wheel discs which were fitted to improve cosmetics and to reduce the time taken to clean the wire wheels after use.