B737-500 CAPT RETURNED TO THE DEP ARPT WITH EXCESSIVE ENG VIBRATION AFTER THE MECHS CHANGED SOME FAN BLADES.

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ON JUN/SUN/04; ACFT X ARRIVED AT ZZZ AND MR X AND I WERE ASSIGNED A 3 SVC AND FAN BLADE CHANGE. I BEGAN THE SVC CHK AND WHEN MR X'S SHIFT STARTED I HAD MOST OF THE OUTSIDE OF THE CHK COMPLETED. WHEN MR X ARRIVED AT THE ACFT WE DISCUSSED OUR WORK ASSIGNMENT; CHANGING A DOUBLE SET OF FAN BLADES ON ENG #1 AND A #1 MAIN TIRE CHANGE. WE CHANGED THE TIRE AND I STARTED THE BLADE REMOVAL WHILE MR X CONTINUED WITH THE 3 SVC. AS I REMOVED THE PARTS TO REACH THE BLADES; MR X STOPPED WORKING THE 3 SVC AND BEGAN GETTING THE 2 BLADE SET BOXES AND UNPACKING THEM. I HAD READ THE MAINT MANUAL BEFORE THE ACFT HAD ARRIVED AND FELT I HAD AN UNDERSTANDING OF THE REMOVAL AND INSTALLATION; INCLUDING THE FACT THAT THE INDIVIDUAL BLADES IN EACH SET WERE TO BE INSTALLED 180 DEGS FROM ONE ANOTHER. AFTER REMOVING THE BLADES AND HANDING THEM TO MR X; THE OLD BLADES WERE PLACED IN THE BOX LIDS; NEW BLADES WERE REMOVED FROM THE BOX AND PLACED ON A LIFT TRUCK BED FOR LUBRICATING PER THE MAINT MANUAL. BEFORE STARTING THE LUBRICATION PROC; WE MARKED EACH BLADE WITH THE FAN POS NUMBER. BECAUSE THE BLADES WERE MARKED WITH INCORRECT POS NUMBERS; I INSTALLED THEM TOGETHER; RATHER THAN 180 DEGS APART. I VERIFIED THAT THE INSTALLATION WAS CORRECT BY BLADE POS NUMBER AND DID NOT RECHK THE BLADE WTS. WE COMPLETED THE BLADE INSTALLATION PER THE MAINT MANUAL. MR X COMMENTED THAT HE WOULD FEEL BETTER IF THE ENG WAS RUN; BUT SINCE THE MAINT MANUAL DOES NOT REQUIRE A RUN FOR 2 SETS OF BLADES; THE RUN WAS NOT PERFORMED. SUPPLEMENTAL INFO FROM ACN 627104: THE ACR HAS INSTITUTED A FAN BLADE TRAINING PROGRAM. AND ALSO LABELED FAN BLADE BOXES WITH PLACARDS TO INSTALL THE FAN BLADE 180 DEGS APART.