Found loose belt due to loose alt pulley bolt on my 300sdl, I know its 8mm but how do I get alt to stop turning to tighten it up? The fan blade behind pulley I believe is what chewed up my last belt, don't know why the guy I had put new vacuum pump and belt on didn't notice. Anyway I decided I could do it myself and try it with alt in car. I've taken off metal fan blade for better access, upgrade on that someday. Any ideas, doesnt seem to be keyway that I can tell to hold fan and pulley in place while tightening. Best I can tell its a friction fit, I just have to figure out how to keep alt from turning. Any help appreciated. Thank You

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86 300SDL