The components of an adjustable light fixture mounting assembly are secured during adjustment of a light in a lighting system. A knuckle joint with a locking taper secures an upper knuckle half to a lower knuckle half. The locking taper secures the adjustment of the knuckle halves, even if the knuckle screw becomes loosened. In a further embodiment, an O-ring seal between a double locking taper seals the knuckle joint from moisture. A rotating locking clamp secures the lower knuckle half to a nipple with a reverse taper on both the lower knuckle half and the nipple, while allowing rotation of the lower knuckle half with respect to the nipple. Stop pins in the lower knuckle and nipple cooperate with an interior profile of the rotating locking clamp to allow 360 degrees rotation of the lower knuckle half with respect to the nipple.