Sierra International 18-3863 Head Gasket The Sierra 18-3863 Head Gasket is interchangeable with the following: GLM 35880, Mallory 9-63855, Mercury 27-96542-1, 27-96281T01, 27-96542, 27-96281, 27-965422. Each Sierra product meets or exceeds the original equipment part it replaces. Sierra Head Gasket Meets or exceeds the original equipment quality Sierra part number: 18-3863 Interchangeable with the following: GLM 35880, Mallory 9-63855, Mercury 27-96542-1, 27-96281T01, 27-96542, 27-96281, 27-965422