Tin foil just takes up the slop, preventing the arm from bottoming out on the shaft. The hole in the arm is slightly smaller at the top. The apparent idea is to embed the splines into the arm when the nut forces the arm onto the spline. Problem is, the nut will eventually loosen, then the spline can turn within the hole in the arm. This action enlarges the hole. Eventually, tightening the nut will have no effect as the nut bottoms out while the arm is still loose. I had this problem a couple of years ago. Tin foil worked for awhile, but I ended up replacing the arm.