It is quite a simple job that anybody with some technical knowledge can do.
The original US diving board front bumper
Tucked US bumper (1,5 inch / 3,5cm)
Remove the front bumper spoiler, do not forget to unplug the cables connected to the bumper first.
Then remove the bumper itself.
Simplest way is to remove the two M14 bolts (22mm head) of the bumper shocks, they are located near the battery and the other one just on the other side of the front. You also need to take out two smaller M8 screws (head 13mm) on the side of each shock.
Then easily pull out the bumper complete with shocks.
# shock shortening information removed, do it safe, do it right, buy new shocks at BMW for the later '88 and '89 models, these shock are shorter. #
Now place the bumper back on to the car and you will find some new issues to tackle.
One of the issues is the cover plate that is on top of the bumper, a gap of a few mm will be visible from the left to the right side of the bumper, not very nice!
When you take off the cover, you will notice that the first inch of the inner surface is trimmed down a bit to give space for the bumper. But as the bumper is now pushed in much further, it will exceed the trimmed edge and therefore the cover will touch the bumper, pushing it upwards.
I cut the trimmed edge much further out to give it more space. This cannot be grinded, you will have to look for another solution. I did it with an electric wood planer which worked out very well.
Another issue you will be facing is a gap between the side bumper trims and the bumper.
As the bumper is pushed further in, the corners of the bumpers are pushed in a bit too, causing them to drive a bit away from the side bumper trims.
For Europeans (like myself) this can easily be solved. I solved it by exchanging the US license plate holder for a European license plate holder of a European Highline (world bumper) model. These fit to these bumpers perfectly and also fit to the original rubber trims.
After installing the much wider license plate holder, the rubber trims can be moved more to the outside (you have to drill some new holes). Now the thicker (front/corner) part of the rubber trim will be moved to the side of the bumper (the rubber is narrower there originally) causing the gap to close completely.
I will add a new post about the rear bumper in a while as well.