Upper radiator hose is the usual choice. If you'd like to measure radiator efficiency, use two and put the other in the lower radiator hose.
Part #4:
http://www.realoem.com/bmw/showparts...67&hg=11&fg=35
Part #4:
http://www.realoem.com/bmw/showparts...67&hg=11&fg=35
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