A BELL 206L-4 WAS DISPATCHED AND OPERATED WITH THE INCORRECT PART INSTALLED IN THE MAIN ROTOR DRIVE.

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ON JUL/XX/98; I DECIDED TO CHANGE THE FREEWHEELING ASSEMBLY ON ACFT XYZ AS IT WAS LEAKING. CALLED PARTS AND ASKED FOR P/N 406-040-500-115. PARTS MAN SAID HE WOULD CHK AND GET BACK TO ME. AN HR LATER I CALLED PARTS TO CHK STATUS. PARTS MAN SAID HE HAD A 406-040-500-123. SINCE THE NUMBERS WERE CLR; I ASKED IF IT WAS A SUITABLE SUBSTITUTE. HE REPLIED THE COMPUTER SAYS ITS A GOOD NUMBER. I INSTALLED THE -123; THE ACFT WAS GND RUN; OPS CHK FLT COMPLIED WITH; AND THE ACFT WAS RETURNED TO SVC. ON JUL/XY/98; I WAS INFORMED IN PERSON THAT THE -123 PART WAS NOT APPROVED. THE ACFT WAS FINISHED FOR THE DAY AND HAD ALREADY LANDED. THE ACFT IS GNDED; THE -123 FREEWHEELING ASSEMBLY HAS BEEN REMOVED. AM WAITING FOR THE -115 ASSEMBLY TO BE DELIVERED IN THE MORNING. THE ACFT FLEW ITS NORMAL CONTRACT TODAY; JUL/XW/98. FLEW 2.8 HRS; WITH 5 STARTS AND 6 TKOFS. PLT RPTED NO PROB. APPARENTLY; QUALITY ASSURANCE PEOPLE WERE NOT SURE OF THE APPROPRIATE PART NUMBER AND HAD TO CALL BELL. USUALLY IF A PART HAS A HIGHER NUMBER (NEWER); AND CAN'T BE FOUND IN THE PARTS BOOK; PARTS IS CALLED TO SEE IF THEY HAVE IT LISTED AS A SUITABLE SUBSTITUTE. THEY HAVE THE LATEST AND GREATEST LISTING. THEIR SOURCE; I BELIEVE THEY USE THE PRICE LIST. IF THEY CANNOT CONFIRM THE PART; BELL MUST BE CALLED.