It depends on the shape of the manifold flange and the mating flange on the exhaust pipe. Some manifolds will work with either gasket, depending on the exhaust pipe they are mated with. If there's a 45 degree bevel at the manifold outlet, the donut gasket can be used- - - - -assuming there's a corresponding bevel on the exhaust pipe. The donut usually seals better than the flat gasket. If the flange is welded to the pipe, you'll need the flat gasket. Donut gasket systems use a sliding flange and a flared end of the exhaust pipe.
Jerry
