Sierra 18-3550 140 Degree Thermostat, Replaces 2324347, MerCruiser 120/140/2.5L L4

The Sierra 18-3550 is a 140 degree F thermostat built for MerCruiser stern drive engines, regulating coolant flow so the engine reaches and holds proper operating temperature. Consistent temperature control protects against overheating and against the fuel-fouling and premature wear that comes from an engine running too cold, making the thermostat one of the most important small parts in the cooling system. The 18-3550 features a white housing and is sized to fit directly into the thermostat housing of the applicable MerCruiser L4 engines.

This thermostat is designed for MerCruiser 120 and 140 model stern drives built around the 153 cid and 181 cid L4 engine blocks, as well as the 2.5L 153 cid L4 engine. It is a direct equivalent for a range of original Mercury/MerCruiser thermostats used across these engine families over their production runs, letting owners replace a worn or stuck thermostat without having to track down the original factory part number.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3550
• Temperature Rating: 140 degrees F
• Color: White
• Fitment: MerCruiser 120, 140, and 2.5L stern drives with 153 cid or 181 cid L4 engines
• Weight: 0.08 pounds
• UPC: 030999315991
• OEM Cross Reference: 2324347, 3377340, B531853, B531854, C853255, 2508261, B450800

Fitment

• MerCruiser 120, 153 cid L4, 1968-1972
• MerCruiser 120, 153 cid L4, 1972-1986
• MerCruiser 2.5L, 153 cid L4, 1987-1989
• MerCruiser 140, 181 cid L4, 1968-1986

Replaces (OEM)

• 2324347
• 3377340
• B531853
• B531854
• C853255
• 2508261
• B450800

Frequently Asked Questions

What temperature rating is the Sierra 18-3550 thermostat? The Sierra 18-3550 is rated at 140 degrees F, which is the standard operating temperature setting for the MerCruiser engines it is designed for.

Which MerCruiser engines does the 18-3550 thermostat fit? It fits MerCruiser 120 and 140 model stern drives with 153 cid or 181 cid L4 engines, as well as the 2.5L 153 cid L4 engine, covering model years 1968 through 1989 across those variants.

Does the 18-3550 replace factory Mercury/MerCruiser thermostats? Yes, the Sierra 18-3550 is a direct equivalent for the original equipment thermostats used on these engines, cross-referencing to OEM numbers 2324347, 3377340, B531853, B531854, C853255, 2508261, and B450800.

What color is the 18-3550 thermostat housing? The Sierra 18-3550 thermostat has a white housing.

How much does the Sierra 18-3550 thermostat weigh? It weighs 0.08 pounds.

Is the 18-3550 the right part for a 140 six-cylinder MerCruiser engine? No. The 140 six-cylinder engine uses a different Sierra part number, 18-3551, so buyers with that engine should order 18-3551 rather than the 18-3550.

Is there a thermostat housing kit that pairs with the 18-3550? Sierra offers a related complete thermostat housing kit under part number 18-3650 for engines where the original housing is no longer available; the 18-3550 is the thermostat itself, sold separately from that housing kit.

What is included when ordering the Sierra 18-3550? The 18-3550 order includes the Sierra 140 degree thermostat unit for MerCruiser 120/140/2.5L L4 engines, identified by UPC 030999315991.

How do I know if I need the 18-3550 versus a different Sierra thermostat number? The 18-3550 is specific to the MerCruiser 120, 140, and 2.5L L4 engines listed in its fitment; other engine models, including the 140 six-cylinder, use different Sierra part numbers such as 18-3551, so matching your engine model and OEM cross-reference number is important before ordering.

Is the Sierra 18-3550 a genuine or aftermarket thermostat? The 18-3550 is a Sierra-branded replacement thermostat manufactured to match the specifications of the original MerCruiser factory thermostats it cross-references, rather than a factory Mercury part.