A B737-400 AFTER TKOF DECLARED AN EMER AND RETURNED TO THE FIELD DUE TO LOSS OF ENG OIL CAUSED BY AN N2 DRIVE PAD COVER NOT REINSTALLED AFTER ENG BORESCOPE.

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ON FEB/XA/98; I WAS ASSIGNED TO AN ACFT B737-400; TO HAVE A BORESCOPE DONE AT THE HANGAR WHICH REQUIRES ACFT TO BE MOVED FROM ONE END OF THE ARPT TO THE OTHER. ACFT ARRIVED START OF THE SHIFT AND THERE WAS A LITTLE RUSH TO GET IT TO HANGAR BECAUSE IT WAS AN EARLY MORNING DEP. OPENING ENG WAS ACCOMPLISHED BY 2 DIFFERENT MECHS; MYSELF AS ONE OF THEM. I BEGAN TO OPEN THE HOT SECTION INSPECTION PLUGS; THE OTHER MECH SAID HE WOULD HELP AND REMOVED THE N2 DRIVE PAD COVER. HE REMOVED AND PLACED IT ON TOP OF THE ENG GENERATOR. BORESCOPE BEGAN AND I MADE SURE THERE WAS ANTI-SEIZE ON ALL THE INSPECTION PLUGS AND WROTE SERIAL NUMBER FOR THE NEW IGNITER PLUGS ON THE PAPERWORK. INSPECTION SAID THEY WERE FINISHED BORESCOPING THE ENG. I BEGAN CLOSING ALL OF THE PLUGS TO THE ENG. ONCE I DID THAT I WENT TO INSPECTION AND ASKED THEM TO LOOK ENG OVER FOR OK TO CLOSE THE ENG. THEY SAID IT WAS OK TO CLOSE. I CLOSED THE HOT SECTION ENG COWLS AND THEN THE OTHER ENG COWLS BELIEVING EVERYTHING WAS ACCOMPLISHED. ROUTINELY I STAMPED THE PAPERWORK OFF. I BELIEVED THE WORK WAS COMPLETED AND THE N2 PAD COVER NEVER CROSSED MY MIND. I BELIEVED INSPECTION LOOKED THE ENG OVER COMPLETELY. I DID NOT SEE ANY PARTS ON THE FLOOR WITH THE OTHER PARTS WE CHANGED. THERE IS NO LEAK CHK REQUIRED AFTER OPENING ENG PORTS AND N2 DRIVE PAD ON OUR PAPERWORK. INSPECTION WAS NOT REQUIRED TO CHK THE N2 PAD COVER ON OUR PAPERWORK. I FEEL COMPLACENCY AND LACK OF INSPECTION INVOLVEMENT LED TO THIS INCIDENT. THIS IS A CRITICAL ITEM AND INSPECTION SHOULD BE INVOLVED; LEAK CHKS SHOULD BE ACCOMPLISHED ANYTIME A CRITICAL PART IS REMOVED OR CHANGED.