During the flight the loosened nut caused increasingly excessive valve clearance eventually allowing collision of the valve head with the rising piston fracturing the valve stem, which then allowed the inlet valve to drop open, resulting in the ignition by the spark plug of the pressurised fuel/air mixture within the inlet manifold and, eventually, the pumping of the ignited fuel outboard of the rocker cover and along the outside of the engine, leading to an extensive fire in the engine nacelle. Due to the fire originating in the induction system, where the supercharged fuel/air mixture was at higher pressure than atmospheric, the heart of the fire was much hotter burning and intense than would be the case in a simple fuel fire.