25mm, one inch, is the spec on steering wheel play. The steering box can be adjusted, but ought to be the absolute last thing you do to take out play. All of your front suspension must be tight first. I've got an '83 240D that I've had since new and the steering box has never been touched and is still very tight. When my steering starts to get a little sloppy, it has always been something in the suspension that is wrong and not the box. You can do a whole lot of damage adjusting a box that doesn't need it. By now the entire front end of your car should have been rebuilt. If it hasn't, then I bet that's where your problem lies.

Len