ON DIFFERENT OCCASIONS; DURING PREFLIGHT OF B767-300 ACFT; A PILOT NOTICES NUMEROUS SCREW TYPE FASTENERS MISSING FROM ENGINE PYLON PANELS. SOME ARE NOT DEFERRABLE.

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I FLEW B767 ACFT X ZZZ-ZZZ1 AND ON PREFLT FOUND 8 FASTENERS MISSING ON THE ENG PYLONS; 4 OF WHICH WERE ON THE LEADING EDGE OF PANELS. 7 DAYS LATER I FLEW THE SAME ACFT ZZZ1-ZZZ AND ON PREFLT FOUND 3 FASTENERS MISSING ON THE ENG PYLONS. THESE FASTENERS ARE BACKING OUT AT AN ALARMING RATE. WHATEVER LOCKING MECHANISM THAT IS SUPPOSED TO HOLD THEM IN IS NOT DOING ITS JOB. SOME OF THE FASTENERS ARE A NO-GO ITEM ACCORDING TO MAINT. I WROTE A SAFETY RPT ON THE SAME SUBJECT ON ACFT Y IN JUNE THIS YR. THIS IS A FLEET WIDE ISSUE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THESE ARE NOT THE STANDARD SCREW FASTENERS THAT HE IS FINDING MISSING ON THE ENGINE PYLON PANELS ON B767-300'S. THESE FASTENERS HAVE A ROUND WASHER UNDER THE SCREW. MOST NOTICED ARE THE RECTANGULAR AND CIRCULAR PYLON PANELS WITH MISSING SCREWS FORWARD OF THE HYD COMPARTMENT ACCESS PANEL DOOR. REPORTER STATED IF MAINT WERE TO DO A QUICK WALKAROUND AS SOON AS THE ACFT ARRIVES AT THE GATE; IT MAY HELP IN GETTING A NON-DEFERRAL MISSING SCREW LOCATED SOONER. MANY STATIONS DO NOT MAINTAIN A PARTS SUPPLY OF SOME OF THESE REQUIRED FASTENERS.