A MOONEY 20J LOST PWR ON CLBOUT DECLARED AN EMER AND DIVERTED DUE TO LOSS OF ENG PWR. CAUSED BY MAGNETO MOUNTING NUTS NOT SECURED.

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ON MAY/THU/04; I REINSTALLED A BENDIX D4LN-3000 MAGNETO SERIAL NUMBER X ON MOONEY 20J SERIAL NUMBER Y AFTER REPAIR BY AN OUTSIDE VENDOR. THE INSTALLATION WAS PERFORMED PER MANUFACTURER'S PUBS AND A GND OPERATIONAL/LEAK CHK WAS PERFORMED WITH SATISFACTORY RESULTS AND NO LEAKS NOTED. THE ACFT FLEW APPROX 35-40 HRS SUBSEQUENT TO THIS INSTALLATION AND LOST PWR ON CLBOUT FROM ZZZ AT APPROX XA00 LCL TIME ON MAY/SAT/04. THE PLT MADE A SUCCESSFUL PWR-OFF LNDG BACK AT ZZZ. THERE WERE NO INJURIES OR PROPERTY DAMAGE INCURRED. I WAS NOT AWARE OF THE OCCURRENCE UNTIL TODAY (JUN/TUE/04). 2 MECHS FROM THE COMPANY I WORK FOR TRAVELED TO ASSESS THE PROB. THEY FOUND THE MAGNETO HAD BECOME DETACHED FROM ITS MOUNTING STUDS. AFTER THE MAGNETO WAS REINSTALLED; THE ENG OPERATED NORMALLY. IT APPEARS THAT I HAD FAILED TO FULLY TIGHTEN THE MAGNETO MOUNTING NUTS AFTER TIMING THE MAGNETO TO THE ENG. THE ACFT WAS SCHEDULED TO FLY THE NEXT DAY AND I FELT PRESSURED TO COMPLETE THE JOB. IT WAS LATE IN THE DAY. I WAS PROBABLY DISTR BY SOMEONE OR THE PHONE. I DIDN'T HAVE ANOTHER MECH INSPECT THE WORK BEFORE RETURNING THE ACFT TO SVC. PREVENTION: DO NOT ALLOW PRESSURE TO COMPLETE THE WORK CAUSE ME TO RUSH. AVOID DISTRS. IF NOT POSSIBLE; GO BACK SEVERAL STEPS OF WORK PERFORMED PRIOR TO THE DISTR. HAVE ANOTHER MECH DOUBLECHK WORK WHENEVER POSSIBLE.