This Heavy Duty Aluminum Servo Saver protects the gears of your servo from impacts that would normally break them.
Includes inserts for Futaba, Hitec, and JR servo splines.

Dimensions:
Hub Diameter: 23.8mm (0.94")
Arm Length (from axis): 21.9mm (0.86")
Arm Width: 17mm (0.67")
Arm Thickness: 02.84mm (0.11")
Arm has five 3.175mm (1/8") mounting holes
Weight: 8grams (0.28oz)

Installed on a HXT12K Metal-Gear Servo.

Servo screw head needs to be 6mm (0.24") OD. HXT12K comes as 6.3mm (0.25").  (You can sand it down to fit.)
Too much slop?
These servo savers come with ~10 degrees of wiggle-room, but your design may not need any.  If you want to tighten things up, you can easily open the servo saver by unscrewing the smaller part below:
        
Use a dremel with a small router bit to decrease the wall length in the main body just enough so that the spring will touch both upper and lower walls when assembled.