A BEECH BARON 55 ON A PREPURCHASE INSPECTION WAS FOUND TO HAVE A CRACKED CYLINDER BLAMED ON VIBRATION CAUSED BY A PT 0625 INCH WASHER FOUND UNDER THE CRANK FLANGE.

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JOB WAS TO REBELT THE AIR CONDITIONING PUMP ON THE L-HAND MOTOR. PROP WAS REMOVED; OLD BELTS REPLACED; AND PROP INSTALLED BACK ON ACFT. ACFT HAD THEN BEEN INSPECTED PER ANNUAL INSPECTION AND ALSO A PREPURCHASE INSPECTION. UPON A SECOND PREPURCHASE INSPECTION; THE MOTOR AT THE CYLINDER WAS FOUND CRACKED. THERE IS A SVC BULLETIN ON THESE MOTORS CRACKING (CASE) AT THE SAME LOCATION. UPON REMOVAL OF THE PROP; THERE WAS FOUND A WASHER (NOT PROP WASHER) THAT HAD BEEN PRESSED INTO THE FLANGE OF THE CRANKSHAFT. WHERE IT CAME FROM AND HOW IT WAS IN THAT LOCATION IS UNKNOWN. I DO NOT KNOW IF THIS WASHER CAME FROM MY INSTALLATION OF PROP OR WORK DONE AFTER. I STILL DO NOT KNOW HOW THE WASHER STAYED IN ITS LOCATION IN A VERT PLANE WHILE PUTTING PROP ON. I DO NOT FEEL AS THOUGH I AM AT ANY FAULT; BUT WAS LAST PERSON TO MAKE ENTRY INTO LOGBOOK FOR PROP REMOVAL AND REPLACEMENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATES THE WASHER WAS NOT INSERTED WHEN THE PROP WAS REINSTALLED AFTER THE AIR CONDITIONING PUMP BELT'S CHANGE. THE RPTR SAID THERE WERE NO RPTS OF ENG VIBRATION AT ANY OF THE INSPECTIONS FOLLOWING THE WORK PERFORMED BY THE RPTR. THE RPTR STATES HE HAS NO KNOWLEDGE OF WHERE OR HOW THE WASHER GOT BTWN THE PROP AND THE CRANK FLANGE.