What was found
PILOT REPORTED THAT ON DOWNWIND, IN TRAFFIC PATTERN AT DESTINATION AIRPORT, HEARD A LOUD "POP," AND EXPERIENCED A POWER LOSS ON THE LT ENGINE. ALSO NOTICED OIL AND SMOKE COMING OUT OF LT ENGINE COWL. SHUTDOWN ENGINE AND FEATHERED PROPELLER IAW ENGINE FAILURE CHECKLIST AND LANDED NORMALLY. VISUAL INSPECTION OF THE LT ENGINE REVEALED A QUARTER SIZED HOLE IN THE CASE NEAR THE FORWARD HOIST RING. ON REMOVAL, A CRACK WAS NOTICED ON THE LT ENGINE CASE SECTION IN FRONT OF THE NR 6 CYLINDER. CRACK EXTENDED THROUGH ENTIRE CASE HALF. ONE OF THE COUNTERWEIGHT DAMPER PIN RETAINER PLATES BROKE INTO PIECES RELEASING THE PIN ALLOWING THE COUNTER WEIGHT TO CONTACT CRANKCASE CAUSING IMMEDIATE STOPPAGE OF ENGINE.