Sierra 18-3549 Thermostat, 120 Degree F, Replaces 803061T1, for Mercury 100/115/125 HP 4-Cylinder Outboards

The Sierra 18-3549 is a 120 degree F thermostat built for Mercury outboard engines, regulating coolant flow so the engine reaches and holds proper operating temperature. It is used as the thermostat component in Mercury 100 HP, 115 HP, and 125 HP 4-cylinder outboard applications, matching the OEM 803061T1 thermostat kit specification for those engines. The thermostat is finished in white and constructed of metal for durability under continuous exposure to engine coolant and heat cycling.

As a direct temperature-control replacement part, the 18-3549 opens at the correct set point to prevent the engine from running too cool (which can cause poor combustion and increased wear) or overheating due to a stuck valve. It is a straightforward drop-in replacement for a worn, corroded, or failed thermostat and is manufactured to meet or exceed the original equipment part it replaces. This is the thermostat only; it does not include a housing gasket.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3549
• Temperature Rating: 120 degrees F
• Color: White
• Material: Metal
• Weight: 0.07 pounds
• Fitment: Mercury 100 HP, 115 HP, and 125 HP 4-cylinder outboards
• OEM Cross Reference: 803061T1
• UPC: 808282243968

Fitment

• Mercury 100 HP, 4 cylinder, serial number B209468 and up
• Mercury 115 HP, 4 cylinder, serial number C100000 and up
• Mercury 125 HP, 4 cylinder, serial number D283222 and up

Replaces (OEM)

• 803061T1

Frequently Asked Questions

What engines does the Sierra 18-3549 thermostat fit? It fits Mercury 100 HP (4 cylinder, serial number B209468 and up), 115 HP (4 cylinder, serial number C100000 and up), and 125 HP (4 cylinder, serial number D283222 and up) outboards.

What temperature does this thermostat open at? The Sierra 18-3549 is rated at 120 degrees F, which is the set point at which it opens to allow coolant flow through the engine.

What OEM part number does the 18-3549 replace? It replaces OEM number 803061T1, the thermostat specification used in the applicable Mercury outboard applications.

Is the Sierra 18-3549 a genuine OEM part or aftermarket? It is a Sierra-manufactured aftermarket replacement thermostat that is built to meet or exceed the original equipment part it replaces.

What material is the 18-3549 thermostat made of? It is constructed of metal, sized to hold up under repeated heat cycling in the engine's cooling system.

How much does the 18-3549 thermostat weigh? The Sierra 18-3549 thermostat weighs 0.07 pounds.

Does the 18-3549 come with a gasket? No, the 18-3549 is the thermostat only and does not include a housing gasket.

What color is the Sierra 18-3549 thermostat? It is finished in white.

Why would I need to replace a thermostat like the 18-3549? A stuck or worn thermostat can cause an outboard engine to run too cool or to overheat; installing a Sierra 18-3549 restores proper coolant regulation at the correct 120 degree F opening point.

Is installation of the 18-3549 thermostat straightforward? Yes, the 18-3549 is designed as a direct replacement thermostat for the specified Mercury 100/115/125 HP 4-cylinder outboard applications.