A B737-300 MECHANIC DESCRIBES HOW HE INADVERTENTLY INSTALLED A SHROUD OVER THE BOLT HEAD INSTEAD OF THE NUT ON THE ELEVATOR TORQUE TUBE SUPPORT.

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ON ENGINEERING CHANGE ORDER THE MECH IS REQUIRED TO INSTALL A SHROUD ON THE NUT SIDE OF 2 BOLTS CONNECTED TO THE ELEVATOR TORQUE TUBE ONE ON THE L SIDE AND ONE ON THE R SIDE OF THE ACFT. THE SHROUD ON THE L SIDE GOES ON THE OUTBOARD SIDE AND THE SHROUD ON THE R SIDE GOES ON THE INBOARD SIDE. I THINK I MIGHT HAVE INSTALLED BOTH SHROUDS ON THE OUTBOARD SIDES LEAVING THE SHROUD ON THE L HAND SIDE PROTECTING THE NUT AS REQUIRED; BUT THE SHROUD ON THE R HAND SIDE PROTECTING THE BOLT HEAD INSTEAD OF THE NUT. IT WAS DISCOVERED WHILE PERFORMING THIS TASK ON ANOTHER ACFT; I REALIZED HOW THE PAPERWORK READ AND MY POSSIBLE ERROR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE NUT ON THE BOLT END IS A SELF-LOCKING TYPE NUT. THIS BOLT IS PART OF THE ACFT SUPPORT STRUCTURE FOR SUPPORTING THE LWR INPUT TORQUE TUBE THAT INCLUDES THE ELEVATOR LH AND RH AFT CONTROL QUADRANTS. THE INSTALLATION OF THESE SHROUDS ARE DONE DURING THEIR HEAVY CHECKS. HOWEVER; ONLY THE -300'S HAVE THE METAL CASTING TYPE SHROUDS ON THE NUT END OF THE BOLTS; WHEREAS THEIR -700 ACFT HAVE THESE SHROUDS ON THE BOLT HEAD AND NUT ENDS. REPORTER ALSO STATED THE SHROUDS ON THE NUT END DO NOT PREVENT THE NUT FROM LOOSENING; ONLY FROM TOTALLY BACKING-OFF AND FALLING OFF THE THREADED BOLT END. THE BOLT HEAD SHROUD HAS THE SAME EFFECT -- TO CONTAIN THE BOLT FROM MIGRATING COMPLETELY OUT. REPORTER ADDS HE DOES NOT KNOW WHY THE -700'S HAVE THE BOLT HEAD SHROUDED WHEREAS THE -300'S DO NOT.