560 SL steering box

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I have had the entire front end of my 86 560 SL gone over by a very good shop, so I am sure that the front is tight. The problem I am having is drift. There seems to be a dead spot about a 3 to 4 inch area on the steering wheel were nothing happens. Once you go beyond that spot the car reacts, but then you get into an oversteer situation. The steering box was replaced about a year ago from the orginal ower. Can I adjust the steering box, if so, how do I do this and how do I know it's not to tight or to loose. This dead space, does not happen why into a turn, then the steering wheel is tight.