Riveted E-609 root rib to spar. Used squeezer. Plans call for 3-3.5 rivets, but after first one, switched to 3-4 rivets, a much better choice for dimpled rivets in this location. Filed end of spar flange to allow WD-605 control horn to fit. Measured and marked centerlines on spar web and horn arms. Clamped horn to spar. Drilled and clecoed horn to spar.