There are three different types of bushings used in BMW front control arms.
- Bushings with smooth stainless steel skins.
- Bushings with somewhat smooth aluminum skins.
- Split rubber bushings with rough aluminum skins.
Why does this matter? It matters because one of the three versions does not cause any damage to the control arm bore when pressed in and out, and the other two do.
Since stainless steel is much harder than aluminum, a bushing with a smooth stainless steel outer skin can be installed and removed from the control without causing any damage to the aluminum control arm bore. The aluminum of the control arm moves out of the way of the much harder steel skin and no metal transfer takes place.
When you press an aluminum skinned bushing into or out of an aluminum control arm, you have two equally soft metals, and neither can force the other out of the way. What happens is some of the aluminum from both the bushing and the control arm gives way to the other and the end result is metal transfer (galling) between the two parts.
See the following BMW bushing images for examples: