4 Cylinder Exhaust Manifold to Head Gasket | Sierra 18-06804 Cylinder Exhaust Manifold to Head Gasket Sierra 18 0680 Combination intake & exhaust manifold gasket fits GM 3. 0L 4 cylinder marine engines with Vortec cylinder heads. Exhaust manifold gasket For GM 3. 0L engines from approximately 1991 and up with Vortec cylinder heads All intake ports are of equal size and shape Will not work with all 3. 0L engines refer to engine model and serial for correct gasket Replaces Mercury 27 815528 Replaces OMC