From Futaba's website...
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Proper Servo Mounting:
The proper way to mount a servo is as follows:
Insert a rubber grommet into each of the four servo holes.
Insert a metal eyelet from the bottom side of the rubber grommet. This way the wide portion of the eyelet will be in contact with the servo tray when mounted.
Test fit the servo in the tray, and enlarge the openings so the servo will not touch the tray. The rubber grommets will isolate the servo from the hard vibration of the airplane's structure.
Position the servo, then mark the location of the mounting holes. Drill pilot holes with a 1/16" bit at each mark.
Use the servo screws supplied with your radio to mount the servo(s) in the servo tray. Tighten the screws until they just touch the top of the metal eyelet.
See also...
http://home.attbi.com/~aeronut/servo_mounting.htm