The large hex nut simply locks the middle screw, which is how the adjustment is made. Typically, you have to use a metric allen wrench for the middle adjustment screw. Put the allen wrench in and note the direction is is pointing. While holding it stationary, loosen the locknut. Now you will be able to turn the middle screw, which will adjust the lash. You turn it in (clockwise) to tighten and out to loosen. Most people will suggest making minor adjustments and then test driving. After you've made your initial adjustment, hold the position of the center screw with the allen wrench and then tighten the locknut down against the steering box housing. That will lock the center screw into position. Typically, if you adjust the center screw down too tight, the steering wheel will not want to center itself after making a turn. I hope this info helps.

Grace and peace,

Robert K
1991 535i