Sierra 18-1971 Elbow The Sierra 18-1971 Elbow is interchangeable with the following: GLM 51120, Mallory 9-40558, Marine Power 0993-001, Mercruiser 76351A04, 76351A2, 76351A1, 60426A1, 49964A2, 49964A1, Osco 6351. Each Sierra product meets or exceeds the original equipment part it replaces. Sierra Elbow Meets or exceeds the original equipment quality Sierra part number: 18-1971 Interchangeable with the following: GLM 51120, Mallory 9-40558, Marine Power 0993-001, Mercruiser 76351A04, 76351A2, 76351A1, 60426A1, 49964A2, 49964A1, Osco 6351