I think BMW did a mistake with the 2 bar expansion cap, and fixed it on the newer BMW models to 1 bar expansion caps to prabably avoid the catastrophic cooling system failures (ruptured radiator necks, split expansion tanks to name a couple). Pretty much for the same engine (N52 vs M54 - not much different).
Edit: The above statement is wrong. The source I quoted was off.
Last edited by doru; 03-07-2013 at 08:51 AM.