Sierra International 18-0389 Heat Exchanger End Gasket The Sierra 18-0389 Heat Exchanger End Gasket is interchangeable with the following: GLM 31830, Mercruiser 27-99960. Each Sierra product meets or exceeds the original equipment part it replaces. Sierra Heat Exchanger End Gasket Diameter: 3-15/16" Sierra part number: 18-0389 Interchangeable with the following: GLM 31830, Mercruiser 27-99960