Sierra 18-3543 143 Degree Thermostat, Replaces BRP 435491, Fits 200-235 HP Outboards

The Sierra 18-3543 is a 143 degree F thermostat engineered for marine outboard cooling systems, controlling engine operating temperature by regulating coolant flow once the engine reaches its target range. Maintaining the correct opening temperature protects the powerhead from overheating damage while allowing the engine to reach efficient operating temperature quickly, which supports fuel efficiency and reduces wear on internal components. The thermostat is built from plastic construction for corrosion resistance in a marine environment and is a direct equivalent for the BRP 435491 thermostat, making it a straightforward replacement during routine cooling system maintenance or when an OEM unit fails. It ships as a single unit ready for installation in place of the original thermostat. This part is applicable to 200 HP outboards from 1977-83 and 235 HP outboards from 1978-85. Boat owners performing scheduled maintenance, addressing an overheating alarm, or replacing a stuck or failed thermostat can use the Sierra 18-3543 as a reliable OEM-equivalent part backed by the Sierra brand, a longstanding supplier of marine engine parts.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3543
• Product Type: Thermostat
• Temperature Rating: 143 degrees F
• OEM Cross Reference: BRP 435491
• Material: Plastic
• Weight: 0.06 pounds
• Package Length: 3.048 centimeters
• Package Width: 2.54 centimeters
• Package Height: 2.54 centimeters
• UPC: 808282265533

Fitment

• 200 HP outboards, 1977-83
• 235 HP outboards, 1978-85

Replaces (OEM)

• BRP 435491

Frequently Asked Questions

What temperature does the Sierra 18-3543 thermostat open at? The Sierra 18-3543 is rated to open at 143 degrees F, which is the standard operating threshold it is designed to maintain.

Is the 18-3543 a direct replacement for the BRP 435491 thermostat? Yes, the Sierra 18-3543 is a cross-reference equivalent for BRP part number 435491.

What outboards is the Sierra 18-3543 thermostat designed for? It fits 200 HP outboards from 1977-83 and 235 HP outboards from 1978-85.

What material is the 18-3543 thermostat made from? This thermostat is made from plastic construction suited to marine cooling system use.

How much does the Sierra 18-3543 thermostat weigh? The 18-3543 weighs 0.06 pounds.

What are the package dimensions of the 18-3543 thermostat? The packaged unit measures 3.048 centimeters long, 2.54 centimeters wide, and 2.54 centimeters high.

Does the 18-3543 thermostat include installation hardware? The Sierra 18-3543 is supplied as a single thermostat unit for installation into the existing housing.

Why does my outboard need the correct thermostat temperature rating? Installing a thermostat with the wrong temperature rating, rather than the 143 degree F rating of the 18-3543, can cause the engine to run too cool or too hot, reducing efficiency and risking overheating damage.

Is the Sierra 18-3543 an OEM or aftermarket part? The 18-3543 is a Sierra-branded aftermarket thermostat manufactured as a cross-reference equivalent to the OEM BRP 435491 thermostat.

What signs indicate my outboard thermostat needs replacing? Overheating alarms, erratic temperature gauge readings, or reduced cooling performance are common signs that a thermostat like the 18-3543 should be replaced.