When the head is off, push some kind of a wire brush, plastic brush or flexible and mildy abrasive tubing down the coolant passages on the block. Try to do it such that it goes in one hole and comes out the other and you can seesaw it back and forth gently. This is the only way to dislodge the rust that has accumulated there. If there's a more effective way to do this, please let us know, this is important. I believe plumbers do use some kinda flexible tubing to push down pipes but have never checked if it can be adapted well to this situation.
After that, try to wash all that rust out before putting that head back. Perform the same procedure on the cylinder head as well. After everything is fixed up, flush the radiator twice, and flush it once a week, every weekend, for the next 3 weekends or for as long as it takes for you to no longer spot rust flakes in your water. Needless to say, use cheap, sweet smelling, brightly coloured antifreeze that you can buy from autozone or walmart for under $5 a can. It works better than bmw's coolant and you can buy several cans for the same price as one bmw coolant bottle.
Last edited by robertobaggio20; 02-02-2011 at 09:44 AM.