[Ford coupe]

Want to try and get some play out of steering at straight ahead steering ..Hope a can get some with the screw on top of steerin g box but when i loosen lock nut i can not turn screw ..I see like a atar washer under the lock nut and sure this is the problem...Could someone explain and tell me if it is safe to do this.The car is a 55 crown vic and just a bit to much play and all is good undeneath