This article describes the tool and method specified in the BMW shop manual for the 1991 and later R100GS/R, plus K75/100/1100 with the threaded sleeve and lockring design.
This system might seem a bit complicated at first but it's vastly superior to the previous design, which generally consisted of an adjusting nut, upper triple clamp and locknut. The problem with the older design is that the threads on the adjusting nut are loaded one way when adjusting, then pushed the other way by the locknut. The lockut adds preload to the bearing, making it too tight if you start off with it feeling 'right.' With the new system, the adjuster is locked by pulling it away from the bearing, which therefore doesn't change the bearing preload.
Tool 31 4 890 is just a simple metal sleeve. It fits over the locking tube so that when the locknut is tightened it does not bear against the upper triple clamp, but rather it just tightens against the knurled adjusting ring. This locks the adjusting ring to the locking tube, allowing both to spin together on the steering tube, which in turn allows you to apply a measured torque to the adjusting ring by torquing the locking tube.
I made the tool with a short section of 7/8" copper pipe, which worked fine on the GS. The critical characteristics are that the ID must fit over the locking tube, the OD must fit inside the triple clamp, and the height must keep the locknut clear of the triple clamp. You want it to be as high as possible so that you don't have to thread the locknut down farther than necessary, but you need to fully engage the locknut's threads.
"To make your own Thrust Collar adjusting sleeve go to your local hardware store and buy a 3/4"X3/4" copper connecting sleeve. This sleeve is normally used to slip over two 3/4" ID copper pipes to allow them to be joined together with solder. The ID and OD of this sleeve work well as a home made thrust collar. Cut a section of the sleeve 10 to14mm. Make sure it is cut evenly or you may have to 'dress' it a little with a file, sandpaper on a hard surface or a grinder to get the edges parallel.
"This is what I used to adjust my K1100LT steering head bearings. I have used this home made adapter on two other late model K-Bikes and it works great. The adapter I made is actually about 1/2" wide and it works great."
OD: 24.55mm, 0.9655"
ID: 22mm, 0.8665"
Height 10mm, 0.396"
(wall thickness 1.3mm, 0.0510")
I found this technique to work very well. I was surprised how much tighter the bearing was when I used this method, compared to what I had previously done "by feel." The whole procedure was very simple, the only trick part is loosening the locknut after setting the bearing torque, without changing the adjustment. Two wrenches are required (not really that tricky, but it's still the hardest part!).
Unfortunately, the reality is that the knurled adjuster will often be frozen to the top clamp by corrosion, requiring complete disassembly of the upper steering head components.
This can be done without removing the tank, but it's not advisable. The tank is easy enough to remove, and doing so protects it from the errant wrench. Also, I didn't use the tape, but it would be useful in ensuring that the adjustment doesn't change while you remove the locknut.
The Nm to ft-lb conversions in the instructions aren't very consistent, but that's what the BMW manual says.
Prior to 1991, these bikes had the older design with a large hex bolt on top of the clamp and a slotted adjusting ring underneath; much like the design of the previous 20 years. The new design is (in my opinion) much easier to adjust, but unfortunately none of the parts (upper and lower clamp, adjusters, etc) are the same. For the R100GS, the conversion parts are well over $1000. For the K75, the new parts don't match the size of the older fork tubes, so there really is no compatibility at all.