What was found
ACFT DECLARED EMERGENCY DUE TO OIL LEAK AND EXCESSIVE VIBRATION. LANDED ON AIRFIELD WITHOUT FURTHER INCIDENT. INSPECTION REVEALED NR 3 CYLINDER BASE WAS CRACKED 75 PERCENT OF THE WAY AROUND AND NEARLY SEPARATED FROM ENGINE. SUSPECT 2 POSSIBLE CAUSES: FWD UPPER BASE NUT WAS LOOSE. POSSIBLY IT WAS NEVER TIGHTENED, BUT MAYBE IT VIBRATED LOOSE. ACFT HAS UNAPPROVED SPINNER INSTALLED WHICH BLOCKS 66 PERCENT OF COOLING AIR FOR OIL COOLER. POSSIBLE OVERHEAT ISSUE.