Drilled 1/8" hole in horn for clevis pin. However, since I did not first verify the distance from the center of the clevis hole to the bottom of the clevis channel it was necessary to file the outside radius orf the horn down about 1mm so that pin could be inserted and clevis could pivot freely.
Lots of lateral tolerance for the horn in the clevis channel so getting the horn exactly on the servo rod center line was not as critical as I originally thought.
