Outboard Thermostat – Fits Yamaha 2-Stroke & 4-Stroke 2.5-250 HP (Replaces 6E5-12411-30, 688-12411)

Outboard Thermostat – Fits Yamaha 2-Stroke & 4-Stroke 2.5-250 HP (Replaces 6E5-12411-30, 688-12411)

This aftermarket outboard thermostat is engineered to regulate engine temperature for a wide range of Yamaha outboard motors. It fits various 2-stroke models from 3 HP to 250 HP and 4-stroke F-series models from 2.5 HP to 100 HP, ensuring your engine operates within its ideal temperature range for optimal performance and longevity.

This component is a direct replacement for multiple OEM and aftermarket part numbers, including Sierra 18-3540 and 18-3622, and Yamaha 6E5-12411 and 688-12411 series. Please consult the cross-reference table to confirm compatibility with your specific part.

• Complete Kit: Includes the thermostat, thermostat housing gasket, and seal for a full replacement.
• Installation Type: Designed for a direct OEM replacement fit.
• Operating Temperature: Rated at 50°C (122°F), primarily for 2-stroke applications.

Part Number Cross-Reference

Use this table to confirm equivalent part numbers across OEM and aftermarket brands.

Compatible Engine Models

Confirm your engine model/year in the table below. For Yamaha owners, here’s a guide to help you identify your engine’s serial/model: Yamaha Serial Number Guide. You can also review exploded diagrams to verify part numbers: Yamaha Parts Diagrams & Catalog.

Yamaha 2-Stroke Models

Yamaha 4-Stroke Models

Technical Specifications

Package Contents

• 1 x Thermostat (Replaces 688-12411, 6E5-12411-30)
• 1 x Thermostat Housing Gasket (Replaces 688-12414)
• 1 x Thermostat Seal (Replaces 688-12412)

Installation Guide

1. Safety First: Ensure the engine is off and cool. Disconnect the negative battery cable.
2. Drain Coolant: Place a drain pan under the engine and drain the cooling system partially to access the thermostat.
3. Locate Housing: Find the thermostat housing on the engine block or cylinder head.
4. Remove Housing: Loosen and remove the bolts securing the thermostat housing cover.
5. Remove Old Parts: Carefully take out the old thermostat and gasket. Note the thermostat's orientation.
6. Clean Surfaces: Thoroughly clean the mating surfaces of the housing and cover, removing any old gasket material or sealant.
7. Install New Thermostat: Insert the new thermostat in the correct orientation.
8. Install New Gasket: Place the new gasket and seal onto the housing.
9. Reassemble: Reattach the housing cover and tighten the bolts to the manufacturer's specified torque.
10. Refill Coolant: Refill the cooling system with the appropriate coolant mixture.
11. Check for Leaks: Reconnect the battery, start the engine, and let it warm up. Carefully inspect the housing for any coolant leaks.

Troubleshooting & FAQ

Q: How do I confirm this thermostat fits my Yamaha engine?
A: Start by checking the compatible model tables on this page. For absolute confirmation, cross-reference your existing part number with the numbers listed in our cross-reference table. Use our Yamaha serial number guide to identify your exact engine model if you are unsure.

Q: What are the signs of a faulty outboard thermostat?
A: Common symptoms include engine overheating at various speeds, the engine taking an unusually long time to reach its normal operating temperature, or erratic temperature gauge readings. A stuck-open thermostat will prevent the engine from warming up, while a stuck-closed one will cause overheating.

Q: What is the operating temperature of this thermostat?
A: This thermostat is rated for 50°C (122°F). This rating is standard for many of the specified 2-stroke applications, but you should always verify the required temperature for your specific engine model.

Q: Does this kit include the required gaskets for installation?
A: Yes, this is a complete kit. It includes the thermostat, the thermostat housing gasket (replaces 688-12414), and the thermostat seal (replaces 688-12412).

Q: OEM vs. Aftermarket?
A: This listing is an aftermarket replacement part (not an OEM component). We build to OEM specifications using comparable-grade materials. Production is done in an ISO9002-certified facility, and each batch is inspected for consistent performance.

Q: How fast do you ship and how much does it cost?
A: We offer fast global shipping—typically 5–15 days worldwide—and standard shipping is free. Expedited options are available at checkout depending on your location.

Q: What is your warranty and return policy?
A: This product includes a 1-year warranty against manufacturing defects under normal use. If something goes wrong, contact our support team with your order details. Returns are accepted within our standard return window (unused & in original condition); please see our policy page or reach out for instructions.

Important Notes

• Verify that the product images and specifications match your original part. If discrepancies exist—even if your model is listed—do not proceed with the purchase.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.

Why Choose JLM MARINE?

• Quality Assured: As a trusted manufacturer for over 20 years, we supply top marine brands globally.
• Fast Global Shipping: Free delivery worldwide in 5–15 days.
• Direct from Manufacturer: Competitive pricing without compromising quality.