You didn't grease the ball joint before assy did you?



In any case, just give the lower control arm a rap with a hammer to force it down slightly to wedge the taper, put a few washers under the nut, eventually eliminating them as you tighten the nut several times and removing a few washers each time untill the taper establishes a tight enough fit for you to fully draw it into the LCA ball joint tapered bore