They are used on the end studs only. they are to pull down on the manifold away from the hole where there is studs. they are to suppose to stop the manifold from cracking or stud Shear when expanding and contracting. After many heat cycles the manifold will shrink. Ask anyone making power with a diesel. Our cummins sheared the end bolts off from the manifold shrinking. We replaced it with a two piece manifold from BD Diesel. They make these just for this reason. $400.00 plus and they sell a **** load. The dealer got us another factory one through warantee. It shrank at least a 1/2 inch end to end. Check out the holes in the ends of your manifold. They are larger in diameter for this reason. If you would put a flat washer on the end studs they could be sunk into the bigger holes preventing movement resulting in a sheared stud. Or a weak one that snaps when yo go to remove the nut. This washer is not a total cure but it helps. Now every one will come up with a reason why this is wrong. But this is the only reason for the washers. Don't use them I don't care . I remove alot of these studs for people. I will start a thread on easy stud removal

 

They are cupped. Raised center goes torward the nut.