Sierra 18-0103 Powerhead Base Gasket The Sierra 18-0103 Powerhead Base Gasket is interchangeable with the following: GLM 30990, Mercury 27-796501, 27-79650, 27-37725, 27-72496. Each Sierra product meets or exceeds the original equipment part it replaces. Sierra Base Gasket Meets or exceeds the original equipment quality Sierra part number: 18-0103 Interchangeable with the following: GLM 30990, Mercury 27-796501, 27-79650, 27-37725, 27-72496