You can search for the diagram on http://getbmwparts.com/

I haven't drained the radiator before on my car so someone else will need to chime in here.

When I have done radiators in the past, I've always cut the hose and drained it. It makes it easier to remove the hose when it's cut in half.

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Taz... My favorite BMW so far.
Sport, Cold weather, Logic 7

19" Miro M6 Wheels w/competition stems
MGP Claiper Covers (BMW)
Dension Gateway 500 and Audio Video Router
AC Schnitzer pedals
Carbon Fiber Di-Noc Trim
5W White Angel Eyes
Polk tweeters