When I did my 1989 740 turbo exhaust, I dermel tooled all the Ex. manifold nuts off on the flat edge of the nut down to the threads. I then use PB Blaster onto the cut edge so the solvent seeps into the threads. make sure all the special washers and spaces go back properly so the new nuts hold down the ex. manifold. If you fail to replace the hardware correctly, the manifold will loosen and blow out the gasket.