First of all, on my WH 540, the bolt that came in the hardware package was 3" long. When you thread the horns on, the center to center distance is about 2 1/2". My servo horn is 3" center to center. So to keep these even, I used a bolt that's 3 1/2" long so as to keep my center to center the same as my servo horn measurment. Make sure your distance's are equal on both ends if your using a pull/pull setup.

Is there 2 holes in the plastic rudder horns? (Memory lapse) If there was, I think I used the inner one and ground off the extra to shorten the horn.