COMPANY MECH USES A RAG TO BLOCK AN OPEN GEARBOX FROM FOD. RAG DISAPPEARS. SEARCH ENSUES; UNABLE TO LOCATE. ACFT DEPARTS AND MUST RETURN LAND DUE LOW OIL PRESSURE WARNING.

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I AM CURRENTLY EMPLOYED AS AN A&P MECH WITH THE AIRLINE AT THE MAINT OP CTR. ON DEC/THU/92 I WAS SENT TO LAX TO DO A JOB. I WAS WORKING ON A WDB #3 ENG. THE JOB CONSISTED OF REMOVING A ROSAN FITTING FROM THE VENT TUBE LOCATION ON THE TRANSFER GEARBOX. I THEN HAD TO DRILL AND TAP THE HOLE TO INSTALL A LARGE THREADED BUSHING. TO MINIMIZE THE THREAT OF FOD ENTERING THE GEARBOX CAVITY I USED A PIECE OF A SHOP RAG AND PETROLATUM GREASE TO BLOCK THE BACK OF THE HOLE. AFTER THE TAPPING OP I REALIZED THAT THE RAG MAY HAVE FALLEN INSIDE. MYSELF AND A LINE MECH FROM LAX TRIED TO FIND THE RAG; WITH NO LUCK. WE DRAINED THE GEARBOX AND STILL DID NOT FIND A RAG. I NOTIFIED THE GRAVE SHIFT FOREMAN AT LAX THAT I MAY HAVE DROPPED A PIECE OF A SHOP RAG INTO THE GEARBOX CAVITY. HE SENT ANOTHER MECH OUT TO BORESCOPE THE INSIDE OF THE GEARBOX LOOKING FOR A RAG. THE MECH AND THE FOREMAN INSPECTED THE INSIDE OF THE GEARBOX AND COULD NOT FIND ANYTHING. THE FOREMAN ASKED ME REPEATEDLY IF I THOUGHT THE RAG WAS INSIDE. I SAID I THINK IT IS. THE REAR COVER WAS ALSO PULLED OFF THE GEARBOX BUT WE STILL COULD NOT SEE ANYTHING. AT THIS POINT THE FOREMAN TOLD ME THAT AS A RESULT OF BORESCOPING THE GEARBOX HE DID NOT BELIEVE THE RAG WAS INSIDE. I WAS TOLD TO FINISH INSTALLING THE BUSHING. UPON COMPLETION OF THE JOB I ASKED IF I SHOULD STAY FOR THE LEAK CHK. THE FOREMAN SAID NO; THAT IT WOULD BE DONE LATER. I SIGNED FOR INSTALLING THE BUSHING AND WAS TAKEN TO A HOTEL. LATER THAT DAY I RETURNED TO SFO. UPON RETURNING I WAS TOLD THE PLANE HAD FLOWN AND HAD TO RETURN TO LAX. I TOLD LCL MGMNT WHAT HAD HAPPENED. THE ENG WAS REMOVED AND SENT TO SFO MAINT OP CTR FOR REPAIR. I WAS ASKED BY LCL MGMNT TO WRITE A WRITTEN STATEMENT BECAUSE THERE WOULD BE AN INVESTIGATION BY THE AIRLINE QUALITY ASSURANCE AND A RPT SUBMITTED TO THE FAA. THE RAG WAS FOUND AND IT HAD CLOGGED THE SCAVENGE PUMP FILTER SCREEN. I WAS INTERVIEWED TWICE BY THE AIRLINE QUALITY ASSURANCE AND THEY WROTE A RPT THAT WAS SUBMITTED TO THE FAA. THROUGHOUT THE COURSE OF THE INVESTIGATION I LEARNED THAT THE PLANE HAD FLOWN FOR APPROX 30 MINS WHEN THE PLTS HAD TO SHUT DOWN THE #3 ENG AND RETURN TO LAX. THE PROB I WAS TOLD WAS A LOW OIL PRESSURE WARNING. THE AIRLINE IS CURRENTLY WORKING WITH QUALITY ASSURANCE; ENGINEERING AND MECHS TO INSURE THAT THIS DOES NOT HAPPEN AGAIN.