What was found
(CAN) THE REAR ENGINE OF A 337G SKYMASTER WAS REPORTED AS RUNNING ROUGH. INSPECTION FOUND THE SPARK PLUGS FOULED, SO THE PLUGS WERE REPLACED. THE ENGINE RAN SMOOTH AND IT WAS RELEASED FOR RETURN TO SERVICE. ON THE SECOND FLIGHT WHEN POWER WAS REDUCED, THE ENGINE STARTED TO RUN ROUGH WITH ABNORMAL ENGINE OIL TEMPERATURE AND PRESSURE. POWER WAS REDUCED AND THE OIL TEMPERATURE WENT OVER THE RED LINE SO THE ENGINE WAS SHUTDOWN. THE AIRCRAFT RETURNED TO BASE. POST-FLIGHT INSPECTION FOUND THE RIGHT SIDE OF THE REAR COWLING COVERED IN OIL. REMOVAL OF THE COWLING REVEALED THE CRANKCASE WAS CRACKED IN SEVERAL PLACES.