During approach to the Armidale circuit a vibration was felt followed by fluctuating power indications on the left engine. The vibration increased, so the left engine was shut down and the propeller feathered. After landing, oil was observed leaking from a hole in the top of the left engine crankcase. Investigation revealed a fatigue failure of the crankshaft which then caused the failure of two connecting rods, one of which punctured the crankcase causing the oil loss. Metalurgical examination of the crankshaft failure revealed that the fatigue crack had initiated from grinding stress cracks in the aft radius of the No. 5 connecting rod journal. Grinding rework had been performed in this area during the previous engine overhaul, 15 hours before the failure.