Mercury – Thermostat Gasket | FourStroke / SeaPro / Verado / Racing Series | Heat-Resistant O-Ring | 887978

Overview

There’s a certain comfort in knowing your engine is running at the exact temperature it should — not too hot, not too cool, just right in that sweet spot where everything feels smooth and dependable. Anyone who spends long mornings trolling or full afternoons cruising knows how quickly peace of mind disappears when something in the cooling system starts acting up. The Mercury Marine part number 887978 Thermostat Gasket might be small, but it plays a big role in keeping your outboard steady and responsive. When this gasket is sealing the system properly, you feel the engine settle into its rhythm, and that’s when boating becomes effortless again.

Key Features

• O-ring style gasket for secure thermostat housing sealing
• Built from durable, heat-resistant material designed to withstand vibration and thermal cycling
• Manufactured to exact Mercury Marine OEM specifications for long-lasting performance
• Helps maintain proper thermostat function and stable engine temperatures
• Designed for dependable sealing in a wide range of FourStroke, SeaPro, Verado, and Racing outboards

Engine & Boat Compatibility

• Fits Mercury 35-40 Jet / 40 / 50 / 60 EFI FourStroke (4-cylinder)
• Fits select SeaPro 40 / 50 / 60 EFI (serial 1C453840 & up)
• Fits 40 / 50 / 60 / Jet 40 EFI FourStroke (1C050252–1C453839)
• Fits 60R FourStroke (1C615064 & up)
• Fits 350 / 400 / 425 5.7L V10 Verado & 350 5.7L V10 SeaPro AMS (3B266064 & up)
• Fits 400R 5.7L V10 Verado FourStroke (3B272954 & up)
• Fits Formula 60 (4-cylinder) FourStroke (1C104574 & up)
• Common on fishing boats, pontoons, runabouts, bay boats, and performance hulls

Performance & Practical Benefits

In real use, a thermostat gasket like this affects far more than you’d expect. When it seals properly, the cooling system stays tight and stable, helping your engine warm up smoothly and maintain an even temperature throughout the day. You’ll notice the difference in how steady the idle feels, how cleanly the throttle responds, and how the engine tone settles into that familiar hum seasoned boaters learn to trust.

This gasket is built to handle heat swings, long hours, and constant vibration — all the things an outboard experiences during a busy season. It resists flattening, cracking, and hardening, which keeps the coolant where it belongs and protects your engine from costly temperature-related issues. Installation is simple and satisfying; the gasket seats cleanly, and once in place, it delivers a secure, leak-free seal that lasts. Whether you’re putting in long days offshore, cruising from marina to marina, or working your engine hard in shallow rivers, this gasket helps maintain the cooling reliability you depend on.

Why Buy OEM

Cooling systems are not the place to experiment with aftermarket shortcuts. A genuine Mercury Marine part like the 887978 is engineered to match your engine’s thermostat housing perfectly, providing the exact compression and seal needed for consistent performance. Aftermarket versions may look similar, but they often use cheaper materials that degrade faster under heat and vibration. Choosing OEM means fewer surprises, fewer leaks, and fewer headaches. It’s the practical, seasoned-boater choice — the one that keeps your outboard healthy for the long haul.

FAQ

Q1: Which engines does the Thermostat Gasket 887978 fit?

It fits a wide range of Mercury FourStroke, SeaPro, Verado, Racing, and Formula 60 engines, including 4-cylinder EFI models and new 5.7L V10 platforms.

Q2: What makes this different from aftermarket gaskets?

The material quality and precision. Mercury designs it to hold shape under heat and vibration, ensuring a tighter, longer-lasting seal.

Q3: How do I know if this is the correct replacement for my engine?

Match your engine model and serial range to the compatibility list above. If your engine is within those groups, this is typically the correct gasket.

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

Yes. The material resists corrosion, swelling, and degradation, regardless of the water conditions you boat in.

Q5: Is installation easy for most boat owners?

Definitely. Most boaters can install it during a thermostat service with basic tools and a little patience.