Abstract: A combustion seal for sealing high pressure combustion gas within at least one cylinder of an internal combustion engine including a cylinder block, a cylinder head having a groove formed in its bottom face, a cylinder liner having a complementary groove formed in its top face, and a O-ring formed from a ductile material and positioned between and contacting each edge of the groove formed in the cylinder head and the complementary groove formed in the cylinder liner, wherein the O-ring is compressed when the cylinder head is secured on the cylinder such that the O-ring has multiple radial restraint points of contact with the edges. When securing the cylinder head onto the cylinder block, the O-ring is positioned between the grooves in the cylinder head and cylinder liner before a compressive force is applied.