Every servo has a slightly different spline pattern. I have two drawers of servo horns for Futaba, Hitec, and JR. Even between these there are differnt sizes and spline patterns. Then I have another drawer that has bags containing servos for about 10 different horn splines with the servos they work on written on the bag. Not using the arm that came with the servo or one specifically made for that servo is stupid. Just plain stupid. I suppose you use the screws for plastic geared servos in the metal gear servos too....

Sorry but you almost lost your helo because of a dumb mistake.... I apologize for the rant....