Mercury – Water Pump Pulley | Cooling System Component | Cast Steel Finish | 865777

Overview

There is something comforting about hearing your engine settle into that smooth, steady rhythm after you push off from the dock. The cooling system plays a huge part in that peace of mind, and the water pump pulley is right at the center of it. When you’re running a boat you love — one that carries your family, your gear, and your weekend plans — you want every part under the hatch to work without hesitation. That’s exactly why this Mercury Marine part number 865777 feels like a quiet but essential companion on the water. It keeps the flow steady, the temperature stable, and your confidence high, no matter how long the day gets.

Key Features

• Drives the engine water pump with smooth, consistent rotation.
• Built from durable, marine-grade cast steel.
• Designed specifically for serpentine belt systems.
• Engineered to maintain correct belt alignment and tension.
• Resistant to heat, corrosion, and vibration in marine environments.

Engine & Boat Compatibility

• Fits 4.5L and 6.2L MerCruiser sterndrive and inboard engines.
• Also compatible with 8.1L and 8.2L engines using serpentine belt systems.
• Designed for both tow-sport boats and sterndrive-powered cruisers.
• Common on runabouts, family cruisers, performance boats, and modern sport boats.

Performance & Practical Benefits

You feel the difference a quality pulley makes long before you ever look at it. When it’s built right, the belt tracks true and the water pump runs with a steady, confident rhythm. There’s no wobble, no chirping, no strange vibration sneaking into your helm. This pulley keeps your cooling system tight and predictable, which means your engine holds its temperature even when you’re running hard or towing heavy loads.

Its cast steel construction stands up to constant tension and hours of heat cycling, so you’re not worrying about deformation or uneven wear. Because it’s engineered specifically for Mercury’s serpentine belt systems, it aligns naturally with the factory brackets — no shimming, no trial-and-error. The result is a smooth, rock-solid rotation that keeps your coolant moving exactly the way it should.

For boaters who run in both saltwater and freshwater, the corrosion-resistant finish is a quiet blessing. It holds up against the harshness of marine air and the inevitable spray that finds its way into the bilge.

Why Buy OEM

A pulley might seem like a simple part, but choosing a genuine Mercury Marine pulley eliminates so many of the problems cheaper alternatives introduce — belt misalignment, noise, wobbling, premature wear. If you want your cooling system to work right the first time and stay reliable season after season, OEM is simply the smart move. It’s engineered to fit, engineered to last, and engineered for real boats in real conditions. It’s what I’d recommend to any fellow boater who wants less wrenching and more time on the water.

FAQ

Q1: Which engines does this pulley fit?

It fits MerCruiser 4.5L, 6.2L, 8.1L, and 8.2L engines with serpentine belts.

Q2: What makes OEM better than aftermarket here?

Perfect belt alignment and superior durability — two things non-OEM pulleys often struggle with.

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

Match your engine model and ensure you’re running a serpentine system.

Q4: Does it work in salt and freshwater?

Absolutely — it’s built for harsh marine exposure.

Q5: Can most boat owners install it themselves?

Yes, with basic tools and access to the front of the engine.