This is a sail servo arm that fits on the S3801 servo FEATURES: black plastic construction INCLUDES: 1 servo arm SPECS: 4 7/8" overall length 3 15/16" & 4 1/4" (from center of hole where attaches to servo to the centers of the linkage holes) 7/8" at widest point (where mounts to servo) 3/8" at narrowest point (furthest away from servo) LAS:7/16/95 COMMENTS: THIS FITS ANY FUTABA SERVO, BUT MAY HURT GEARING ON HIGH-TORQUE APPLICATIONS. Since the S125 has a square spline this arms will not fit without an adapter, which we don"t carry at this time. UPDATE MCW 3/31/03