ACR MLG UNABLE GET GEAR DOOR CLOSED AND GEAR SAFE INDICATION AFTER TKOF. CAUSED BY FAULTY MAINTENANCE REPLACEMENT OF GEAR DOOR OPERATING ROD CONNECTION PIN AFTER MAINTENANCE.

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I ARRIVED A LITTLE EARLY TO WORK; SO I PERFORMED THE COMPLETE ORIGINATING PREFLT MYSELF WITHOUT THE FO. TAXI AND TKOF WERE NORMAL UNTIL LNDG GEAR RETRACTION; WHEN THE L MAIN GEAR WOULD NOT INDICATE UP; AS DID THE NOSE AND R MAIN GEAR. THE ACFT EXPERIENCED A SLIGHT YAW; SO WE KNEW THAT THE UNSAFE INDICATION WAS VALID. I DECIDED TO ATTEMPT EXTENDING THE GEAR AND RETURN TO LAX; WHICH WE DID USING NORMAL PROC. AFTER LNDG AND RETURNING TO THE GATE; OUR MAINT PEOPLE FOUND THE L MAIN GEAR DOOR NOT ATTACHED AT THE BOTTOM PART AND FREE TO HINGE AT THE TOP. THERE IS A LONG BOLT AT THE BOTTOM PART OF THE DOOR WHICH IS SUPPOSED TO ALLOW HINGING THE DOOR UP FOR MAINT PURPOSES; IF REMOVED. THE BOLT WAS IN PLACE; AS REQUIRED ON THE EXTERIOR PREFLT; WITH ACCOMPANYING NUT AND SAFETY CLIP; BUT NOT PASSING THROUGH THE REQUIRED HOLE ON THE LOWER DOOR. WHEN THE DOOR AND LINKAGE ARE PROPERLY ATTACHED; THE BOLT PASSES THROUGH 3 HOLES WHEN LINED UP; WITH THE DOOR PART HIDDEN BTWN FORWARD AND AFT LINKAGE PARTS. WHEN VIEWING THE ASSEMBLY; AS I DID ON MY PREFLT; IT IS EXTREMELY DIFFICULT TO DETERMINE; JUST BY LOOKING; IF THE BOLT HAS BEEN INSTALLED THROUGH ALL 3 REQUIRED HOLES. IT IS EASY TO ASSUME THAT IF THE BOLT IS THERE WITH A NUT AND SAFETY CLIP; EVERYTHING IS PROPERLY ASSEMBLED. ALSO; THE ACFT LOGBOOK HAD THE AIRWORTHINESS CERTIFICATION SIGNED OFF IN 6/88. THE LOG INDICATED THAT THE L MAIN BRAKE COOLING FAN WAS INOP AND ON THE MEL. I ASSUME THAT SOME MECH HAD DISCONNECTED THE LOWER PART OF THE GEAR DOOR DURING THE PREVIOUS NIGHT; WHILE ATTEMPTING TO REPAIR THE BRAKE FAN; AND DIDN'T GET THE 3 HOLES LINED UP IN STOWING THE LOWER DOOR. I FEEL THAT THE ONLY WAY I WOULD HAVE DETECTED THIS WOULD HAVE BEEN TO GRAB THE BOTTOM OF THE DOOR AND ATTEMPT TO PULL IT OUT. THAT IS NOT NORMALLY DONE ON PREFLT WALKAROUNDS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: GEAR DID NOT OPERATE AND WAS UP; BUT SINCE BIG GEAR DOOR WAS HANGING OPEN COULD NOT GET GEAR SAFE INDICATION AS IT TAKES BOTH GEAR UP AND GEAR DOOR CLOSED TO EXTINGUISH GEAR UNSAFE LIGHT. IS BIG DOOR; SO CAUSED CONSIDERABLE DRAG. THERE IS RED LINE PAINTED ON GEAR DOOR ATTACH POINT TO OPERATING ROD; BUT IS NOT EASY TO SEE FROM THE DECK. PLT HAS HEARD OF SAME OCCURRENCE TO ANOTHER PLT IN LAST COUPLE OF MONTHS. FAA HAS BEEN TO MAINT TO INVESTIGATE INCIDENT; BUT HAS NOT CONTACTED PLT.