What was found
CLIMBING THROUGH 4,000 FEET OUT OF PDX, PILOT NOTED PROMINENT VIBRATION. PILOT TRIED TO DETERMINE WHICH ENGINE WAS SOURCE OF VIBRATION, BUT WITHIN 30 SECONDS, THE LEFT ENGINE STOPPED PRODUCING POWER (DID NOT SEIZE). PROPELLER WAS FEATHERED AND AIRCRAFT RETURNED TO PDX. ENGINE TEAR-DOWN REVEALED THAT NR 5 CONNECTING ROD HAD COME LOOSE. SUSPECT CAUSED BY THE BREAKING OF ONE OF THE CONNECTING ROD BOLTS.