BMW E34 lower control arms, steel vs aluminium
What is the difference between the steel and aluminium lower control arm on the E34?
The aluminium arm is lighter and reduces unsprung weight, and is usually also more expensive. The steel arm is heavier. A difference in durability has not been confirmed. The aluminium arm does not fit all models. The left steel arm is 31121139991 and the left aluminium arm is 31121139987. If you replace both sides, choose matching ones.
Can the lower control arm bushing or ball joint be replaced separately?
The steel arm's bushings can be replaced, the aluminium arm's cannot, because its bushing is physically a different size. The E34 lower ball joint is not sold separately, so in practice the whole control arm is replaced. The ball joint can be pressed out, but it is unlikely to come out intact.
How do you replace the E34 lower control arms yourself?
The lower end of the control arm comes off the strut, so the spring or brake does not need to be removed. The job takes about a couple of hours if the bolts are not seized. The hardest part is removing the lower ball joint, for which a pickle fork helps. The bushings are given their final tightening only when the car is on its wheels.