120 Degree Thermostat | Sierra 18-3549 Aftermarket thermostat used on many different Mercury and Mariner outboard motors. Fits Mercury 6 HP, 8 HP, and 9.9 HP 2 cylinder outboard engines from serial number A197112 and later Fits Mercury and Mariner Sea Pro 10 outboard engines from serial number D038000 and later Fits Mercury and Mariner 18XD, 20XD, and 25XD outboard engines from serial number B114231 and later Fits Mercury and Mariner 25 Sea Pro 2 cylinder outboard engines from serial number B238464 and later Fits Mercury and Mariner 30 HP 2 cylinder outboard engines from serial number G055314 and later Fits Mercury and Mariner 45 Jet, 50 HP, 55 HP, and 60 HP 3 cylinder from serial number G290585 and later Replaces Mercury 14586 Replaces Mallory 9-43014