The flange on the nut you are referring to goes under the bracket. The idea is that it pushes the tensioner pulley into position after the belt is installed. If I remember correctly you should have no tension on the adjuster when installing the belt. Once the belt is in place, turn the nut clockwise, which will "unscrew" it off the threaded rod because it's left hand thread, and that will push the tensioner wheel into position and tighten up the belt. Watch the pointer on the tensioner to determine when to stop turning the nut. Then make sure you tighten down the 19mm bolt that runs through the center of the tensioner.

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Fourings

84 Audi Coupe GT with Turbo engine, EFI (buh-bye CIS), Jamex seats, Koni's/H&R's, e-code lights. 210k and counting.
1992 Anthracite 400E, 161k, "Grey Ghost".
1986 VW Quantum Syncro, 192k, gone to the crusher in the sky.
1989 300TE 172k. (Was mine, now also Mom's car)
72 280SE 4.5 (Mom's car)