Thermostat – Fits Yamaha 2-Stroke Outboards 3-250 HP (Replaces Yamaha 6E5-12411-02-00, Sierra 18-3540)

Product Details

This replacement Thermostat – Fits Yamaha 2-Stroke Outboards 3-250 HP is engineered to regulate engine temperature for optimal performance on a wide range of Yamaha outboards from 1984–2009. It ensures your engine operates within its designed temperature range, preventing overheating and improving fuel efficiency.

It serves as a direct replacement for numerous Yamaha, Sierra, and Parsun part numbers, including the common 6E5-12411-02-00 and Sierra 18-3540, ensuring broad interchangeability.

• Designed for direct-mount installation to the engine block.
• Opens at a consistent 50°C (122°F) for reliable cooling system operation.
• Package includes the thermostat and a corresponding seal for a complete installation.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket.

Engine & Model Compatibility

Confirm your engine model/year in the table below.

Fitment covers a wide array of Yamaha 2-stroke carbureted and HPDI models from 3 HP to 250 HP. Applications for Parsun models are also listed.

Technical Specs

Product Type

Thermostat

Installation

Direct replacement

Fuel Type

Gasoline

Connection

Direct mount to engine block

Housing Material

Brass/stainless steel

Temperature Rating

50°C (122°F) opening temperature

Operating Temperature

50°C (122°F) opening

Warranty

Varies by manufacturer - typically 1 year

Other Technical Attributes

For closed cooling system

What’s in the Box

• 1 x Thermostat (6E5-12411-02-00)
• 1 x Thermostat Seal

Installation Notes

1. Ensure the engine is completely cool before starting. Disconnect the battery.
2. Locate the thermostat housing on the engine block or cylinder head.
3. Place a drain pan under the housing and remove the housing bolts to release any trapped coolant.
4. Remove the old thermostat and its gasket or seal. Note its orientation.
5. Clean the mounting surfaces of the housing and engine block thoroughly.
6. Install the new thermostat and seal in the correct orientation.
7. Re-install the housing cover and tighten the bolts to the manufacturer's specified torque.
8. Reconnect the battery, refill the cooling system, and check for leaks after starting the engine.

Troubleshooting & FAQ

Safety & Important Info

• Verify images/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.
• This is an aftermarket replacement, not an OEM component.
• Professional installation is recommended for safety and performance.

About Sea Sierra

• Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
• Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
• Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.