Futaba sail servo arm's are constructed of black plastic. This fits any Futaba servo, but may hurt gearing on high-torque applications. Since the S128 has a square spline this arms will not fit without an adapter.
Overall Length: 4 7/8"
Length (from center of hole where attaches to servo to the centers of the linkage holes): 3 15/16" & 4 1/4"
Length at widest point (where mounts to servo): 7/8"
Length at narrowest point (furthest away from servo): 3/8"