Thermostat Kit – Fits Mercury/Mariner 70–125 HP 2-Stroke (Replaces 803061T1)

Thermostat Kit – Fits Mercury/Mariner 70–125 HP 2-Stroke (Replaces 803061T1)

This thermostat kit is designed for Mercury/Mariner 70–125 HP 2-Stroke outboards (1987 and newer), including 3-cylinder models (70, 75, 80, 90 HP) and 4-cylinder models (100, 115, 125 HP). It helps maintain the correct engine operating temperature for optimal performance and fuel efficiency.

This kit serves as a direct replacement for several OEM and aftermarket part numbers, ensuring a precise fit and reliable function.

• Engineered for gasoline marine engines.
• Functions as a direct OEM replacement for straightforward installation.
• The complete kit includes a 50°C/120°F thermostat, gasket, and seal.

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 Mercury owners, here’s a guide to help you identify your engine’s serial/model: Mercury Serial Number Guide. You can also review exploded diagrams to verify part numbers: Mercury Parts Diagrams & Catalog.

Technical Specifications

Package Contents

• 1 x Thermostat (Replaces 18-3549)
• 1 x Thermostat Gasket (Replaces 18-0182)
• 1 x Thermostat Cover Gasket (Replaces 18-0339)

Installation Guide

1. Safety First: Ensure the engine is off and cool. Disconnect the battery cables to prevent accidental starting.
2. Drain Coolant: Place a container under the engine and carefully drain the cooling system to a level below the thermostat housing.
3. Locate Housing: Identify and access the thermostat housing on the engine block.
4. Remove Housing: Unscrew the bolts securing the thermostat cover. Gently tap it if it's stuck, but avoid prying, which can damage the surfaces.
5. Extract Old Parts: Remove the old thermostat, gasket, and seal. Take note of the thermostat's orientation.
6. Clean Surfaces: Thoroughly clean the mating surfaces of both the housing cover and the engine block. Remove all old gasket material and corrosion.
7. Install New Kit: Place the new thermostat into the housing in the correct orientation. Install the new gasket and seal as provided in the kit.
8. Reassemble: Re-attach the housing cover and tighten the bolts to the manufacturer's specified torque. Do not overtighten.
9. Refill and Check: Refill the cooling system. Reconnect the battery, start the engine, and let it warm up, checking carefully for any leaks around the housing.

Troubleshooting & FAQ

Q: How do I confirm this thermostat kit will fit my specific engine model?

A: The most reliable method is to check your engine's serial number against the ranges listed in our "Compatible Engine Models" table. Additionally, you can verify fitment by matching your existing part number to those in the "Part Number Cross-Reference" list.

Q: What are the common signs of a faulty thermostat?

A: A failing thermostat can cause the engine to overheat at low speeds or run too cold at higher RPMs. You may also notice erratic temperature gauge readings or an increase in fuel consumption. This 120°F / 50°C thermostat is critical for maintaining correct engine temperature.

Q: How often should I replace my outboard's thermostat?

A: Thermostats are a routine maintenance item and should be inspected annually and replaced every 2-3 years, or sooner if you operate in saltwater. A stuck thermostat can cause severe engine damage.

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.