The pilot reported that soon after takeoff from Cairns the right engine rapidly lost oil pressure accompanied by a rise in the oil temperature. There was also a noticeable drop in manifold pressure and engine rpm. The pilot feathered the propeller and advised the tower of his intention to returned to Cairns where a single engine landing was carried out. A local Licensed Aircraft Maintenance Engineer (LAME) was then engaged to investigate and rectify the defect. After checking the oil quantity and completing a visual inspection of the engine bay the LAME carried out a ground run to verify the extent of the oil pressure loss. All engine parameters including oil pressure and temperature indicated normal during the prolonged ground run. The LAME then advised the pilot that he considered the aircraft to be serviceable and explained to him that the probable cause of the pressure loss was that a piece of carbon had become temperarily lodged under the seat of the oil pressure relief valve. No further "trouble shooting" was carried out by the LAME. Soon after takeoff on the next flight the pilot noticed that the right alternator was not showing a charge. This defect was entered on the maintenance release on arrival in Townsville. The subsequent inspection of the aircraft discovered that the alternator drive "clutch spring" was badly distorted and had caused extensive scoring damage to the internal crankcase casting. The metal filings produced by this scoring action had contaminated the engine oil system. The extent of this contamination necessitated a bulk strip of the engine and accessories. It was found that the crankshaft all main bearings conrod bearings oil pump and oil pump housing had been substantially damaged.