The probable cause of the undercowl in-flight engine fire was the insufficient installation torque of the bolts that secure the fuel manifold cover to the fuel manifold. Engine vibrations and fuel pressure cycles caused the bolts to gradually loosen further until one bolt lost all its tightening torque and fell out. The internal fuel pressure then forced open the fuel manifold cover at the location of the missing bolt, causing a gap between the two mating surfaces which allowed fuel to push past the integral packing, resulting in a fuel leak onto the hot engine cases where it ignited resulting in a fire.