I fitted a non-M alternator on my M3 this week. The one in my M3 died and I'm conveniently parting out a 325ci. The alternator fits, but you need to brake off some of the plastic air duct. Non-M alternators have this air duct but M3 ones do not. You will find that if you do not brake some of the plastic off, it will hit your power steering reservoir. I wasn't comfortable with that and was in a hurry to get the car running, so I took a hammer to the plastic.
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