My M5 is a UK RHD model. This means that the steering box is buried partly under the exhaust manifold and consequently difficult to get at. LHD should be easier in terms of access. The link below should bring up a diagram of the box engineering, and a very good description of how it works.
You will need:
About 3 hours
Ramps or a jack to allow you to get working access underneath the front cross-member.
10 mm, 13mm and 17mm open/ring spanners
5mm allen key, preferably the type which fits into a ratchet drive. Mine does, and can then be turned with the 10mm spanner.
An old rear view mirror, as slim as possible. Mine came from a plastic accessory type.
Masking tape and a magic marker.
Short length (about 18 inches) of dowel or 1/2"x1" timber, or stiff tube.
An assistant for 2 sessions of about 15 minutes each.
Step 1: with the wheel in straight ahead position, cover 3 inches of the wheel with masking tape starting at 12 oíclock and working round, it doesnít matter which way. Put an inch or so of tape on the instrument glass or top of the dashboard lined up with 12 oíclock on the wheel. Make a reference mark on this tape, at 12 oíclock. Now mark the tape on the wheel in segments labelled 0,1,2,3 and so on with "0" lined up to your fixed tape mark. The spacing should be about 1/2 inch apart, but thatís not critical.
Now you have a dead ahead reference point when 0 on the wheel lines up with the fixed mark. Move the wheel through its arc until you feel the system tighten up and note which number segment that happens at. This is a personal judgement, and needs to be repeatable by you.
Step 2: experiment to find a point where you can press the dowel against the body or underside, with one end of it just touching the tyre or wheel rim. The idea is to be able to feel movement in the wheel/tyre amplified through the dowel when your assistant turns the steering wheel.
Step 3: Get your assistant to turn the wheel whilst you have the dowel locked in your chosen place. When you detect movement, ask your assistant which number the steering wheel has reached. This movement again is a bit judgemental, and needs to be repeatable by you.
Step 3: there are 2 heat shields which stop you getting access to the steering box adjustment screw, which is on top of the box. Dealing with the upper shield first, remove the 4 set screws (10mm spanner) which hold the upper heat shield to the top of the exhaust manifold. Remove the set screw (13mm spanner) which holds the bottom of this heat shield to the bottom of the steering box. You should now be able to push the heat shield up to increase the working clearance above the steering box. You might have to jam it in place to keep the clearance.
Now tackle the lower heat shield. It is held in place by one tab which runs to the top (rear)of the steering box, one tab which runs to the back of the box and can be seen from under the car, and one tab which runs to the top (front) of the box just under the manifold. This last tab canít be seen from above. You can just make it out from underneath and can get to it with slim hands. The mirror starts to be useful here to get an idea of where this tab is and it reflects light in under the manifold. All 3 tabs are retained by nuts on studs (13mm). Flex the tabs to get them off the studs and wiggle the shield backwards. I didnít manage to remove this shield, but found I had created enough working space by moving it rearwards.
Step 4: now, with the mirror, you can see the adjustment screw and locknut. Loosen the locknut (17mm spanner) and back it off a couple of turns. Now, use your slim hands to get the allen key into the key way on top of the adjustment screw. I found this is best done from underneath, but itís a personal judgement on the contortions required. The allen key stays nicely in place, allowing you to get the 10mm spanner on it Ė from above, using the mirror.
Step 5: adjust the screw using the allen key. Turn it clockwise to take up the movement Ė you will feel the adjuster tighten up. My car has done 112k miles and took about 1/3rd turn to tighten up. I turned the adjuster about another 1/3rd which was probably just a bit too much in hindsight. As you are making these adjustments, use the reference point on the wheel and with the dowel to get an idea of the difference you are making. I found that the steering wheel slack was taken up before I had reached "1" on my scale and that I could sense the tyre move as soon as my assistant turned off centre.
Step 6: no surprise that itís time to reverse all of the previous operations! Leaving the heat shields held with just 1 or 2 fasteners would allow you to make further adjustment after a road test.
My impressions after a relatively short period with the new set up is of a worthwhile improvement in straight line sensitivity to steering inputs, with a very slight notchiness which I think is caused by me overdoing the clockwise turn. That may bed down after some more miles.