Mercury – 140° Thermostat | Cooling System Series | Stainless Steel Construction | 8M0162139

Overview

If you’ve run big Mercury outboards long enough, you start to appreciate the simple parts that quietly keep everything in balance. The thermostat is one of those unsung heroes. It doesn’t roar, it doesn’t shine, and you’ll probably never brag about it at the dock — but you feel its importance every time the engine settles into that smooth, steady rhythm you trust. Mercury Marine part 8M0162139 brings that same sense of confidence. Whether you’re easing through a flat channel at sunrise or pushing hard across open water, knowing your cooling system is holding temperature exactly where it should be makes the whole day feel easier.

Key Features

• Genuine 140-degree Fahrenheit thermostat
• Fits select 175–300 horsepower and 450R 3.4L V6 and 4.6L V8 four-stroke outboards
• Corrosion-resistant Stainless Steel components for long service life
• Self-cleaning design helps flush contaminants off closing surfaces
• Calibrated to maintain precise engine temperature for peak performance
• Easy installation with basic tools (seals or gaskets may be required)

Engine & Boat Compatibility

• For Mercury 175–300 horsepower four-stroke outboards
• For Mercury Racing 450R 4.6L V8 outboards
• Applicable to 3.4L V6 and 4.6L V8 platforms with serial numbers 2B529482 and above
• Ideal for offshore center consoles, bay boats, bass boats, pontoons, and multiengine setups
• Suitable for vessels that run long distances or operate in varied water temperatures

Performance & Practical Benefits

A thermostat might seem like a small component, but in a high-performance Mercury outboard, it plays a huge role in how the engine feels and performs. This 140-degree thermostat keeps the cooling system in that perfect zone — not too cool where efficiency drops, and not too warm where things start feeling strained. You’ll notice it in the steady idle, in the crisp throttle response, and in the way the engine stays calm when you jump from slow cruising to high-speed runs.

The Stainless Steel construction isn’t just for show; it stands up to constant exposure to salt, minerals, and whatever else the cooling channels pick up. The self-cleaning feature is a smart touch, helping flush debris off the closing surfaces so the thermostat can open and close smoothly. That means fewer surprises, fewer overheating scares, and a cooling system that stays more predictable over time.

Installation is straightforward and doesn’t require specialized tools. You pull the old thermostat, clean the surfaces, drop the new one in, and let the engine do what it was built to do. Once it’s in place, you’ll feel that familiar steadiness return — the engine warming up the way it should, holding temperature, and responding like a well-tuned machine.

Why Buy OEM

With cooling components, precision matters. Aftermarket thermostats can run a few degrees off, cycle inconsistently, or corrode early, all of which affect your engine’s performance and longevity. A genuine Mercury Marine thermostat is engineered specifically for Mercury cooling systems, calibrated to open at the exact temperature your engine was designed for.

OEM gives you predictable performance, longer life, and the kind of reliability that keeps high-horsepower engines running at their best. It’s the practical choice for anyone who treats their equipment with respect — the kind of advice a seasoned boater would pass on without hesitation.

FAQ

Q1: Which engines does the 8M0162139 thermostat fit?

It fits select 175–300 horsepower Mercury four-stroke outboards and the Mercury Racing 450R V8 (serial numbers 2B529482 and above).

Q2: What makes this thermostat better than aftermarket versions?

It’s Stainless Steel, self-cleaning, precisely calibrated, and designed to match Mercury’s cooling specs exactly.

Q3: How do I know if it’s the right replacement?

Check your engine’s serial number and confirm it uses a 140-degree thermostat matching this part number.

Q4: Can it be used in both saltwater and freshwater?

Yes — its corrosion-resistant construction handles both environments very well.

Q5: Is installation straightforward for most boat owners?

Yes. Most owners with basic tools can replace it easily, though new seals or gaskets may be needed.