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The correct fasteners for the manifolds on a straight 6 are studs. Make sure that all are the same diameter and thread. Except for the two long thin ones that hold the thermostat housing in place.
The correct nuts on the studs are a deformed copper alloy nut. BMW, VW and mercedes all use the same nuts. There should be a single washer between the nut and manifold.
My guess, based on how you described the hardware, is that one or more fasteners has been replaced and the threads could be the wrong size (SAE vs Metric or fine vs coarse). Like Mike said, don't force anything.
-CTH
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