Sierra 18-3551 Marine Thermostat for 140 HP 6-Cylinder Engines, Replaces 1538930 1725960

The Sierra 18-3551 is a marine engine thermostat built as a direct replacement for 140 HP 6-cylinder marine engines. It regulates coolant flow through the engine block, opening at 142 degrees F to keep raw water cooled engines running at the correct operating temperature and preventing the overheating or over-cooling that can damage cylinder heads, gaskets, and exhaust components over time. The thermostat is constructed of a metal and plastic composite housing designed to hold up under continuous marine duty in a raw water cooling system.

This part directly replaces OEM numbers 1538930 and 1725960, making it a straightforward swap for boat owners maintaining or rebuilding a 140 HP 6-cylinder marine powerplant. At just 0.12 pounds, it is a compact, easy-to-handle component that fits into the factory thermostat housing without modification. Sierra manufactures the 18-3551 to match the fit, form, and function of the original equipment thermostat it replaces, so cooling system performance and engine temperature regulation remain consistent with the factory setup.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3551
• Product Type: Marine Thermostat
• Weight: 0.12 pounds
• Temperature Rating: 142 degrees F
• Material: Metal/Plastic Composite
• UPC: 030999316004
• Engine Fitment: 140 HP 6-Cylinder engines
• OEM Cross Reference: 1538930, 1725960

Fitment

• 140 HP 6-Cylinder marine engines

Replaces (OEM)

• 1538930
• 1725960

Frequently Asked Questions

What engines does the Sierra 18-3551 thermostat fit? The Sierra 18-3551 is designed for 140 HP 6-cylinder marine engines.

What temperature does the 18-3551 thermostat open at? It is rated to open at 142 degrees F, which is the standard operating temperature for the raw water cooled marine engines it services.

What OEM part numbers does the Sierra 18-3551 replace? This thermostat replaces OEM numbers 1538930 and 1725960.

How much does the Sierra 18-3551 thermostat weigh? The Sierra 18-3551 weighs 0.12 pounds.

What material is the 18-3551 thermostat made of? Sierra 18-3551 uses a metal and plastic composite construction for the housing and internal components.

Is the Sierra 18-3551 a genuine Sierra part or generic aftermarket? The 18-3551 is manufactured by Sierra as a direct aftermarket replacement engineered to match the fit and function of the original 1538930 and 1725960 thermostats.

What is included when ordering the Sierra 18-3551? Ordering the Sierra 18-3551 includes one thermostat unit.

Will the 18-3551 work in a closed cooling system? The Sierra 18-3551 is intended for engines using raw water cooling; confirm your engine's cooling configuration before installing.

Does the Sierra 18-3551 include a gasket or housing? The Sierra 18-3551 is the thermostat itself; the thermostat housing and gasket are separate components.

How do I know if I need the 18-3551 versus a different Sierra thermostat? Confirm your engine is a 140 HP 6-cylinder model and cross-check your original thermostat against OEM numbers 1538930 or 1725960 before ordering the 18-3551.