Mercury – 300 Hour Maintenance Kit | FourStroke Service Package | Complete Engine Care Components | 8M0120839

Overview

There’s a certain kind of confidence you get when you know your outboard is properly cared for—the kind that lets you run a little farther, stay a little longer, and push through whatever the day brings. When you’re depending on a 25–30 horsepower Mercury FourStroke, that confidence comes from routine upkeep done the right way. The Mercury 8M0120839 300 Hour Maintenance Kit delivers exactly that feeling.

Instead of gathering parts piece by piece, this kit puts every essential component for your 300-hour service into one clean, complete package. It’s built for boaters who want their engine to start quickly, idle smoothly, and keep climbing through the powerband without hesitation. Whether you fish the marshes before dawn or cruise the lake with the family on weekends, this kit helps keep your engine performing with the same steady reliability you’ve come to expect.

Key Features

• Complete 300-hour service kit for Mercury 25–30hp FourStroke outboards.
• Includes oil filter, fuel filter, spark plugs (three), oil drain seal, thermostat and gaskets, gearcase seals, unit anode, and water pump impeller kit.
• High-pleat oil and fuel filters deliver superior filtration for cleaner, healthier engine operation.
• Heavy-duty electroplated housings resist corrosion in harsh marine environments.
• Precision-built components engineered to exact Mercury Marine specifications.
• Works seamlessly with Mercury 4-Stroke Marine Engine Oil and Premium Gear Lube (sold separately).

Engine & Boat Compatibility

• Fits Mercury 25–30hp FourStroke outboards
• For engines within serial numbers 0R106999 through 1R162636
• Ideal for skiffs, jon boats, inflatable tenders, aluminum fishing rigs, and utility boats
• Suitable for freshwater and coastal boating environments

Performance & Practical Benefits

A 300-hour service can completely refresh the way your engine runs. Clean filters help fuel and oil move freely, giving you stronger throttle response and smoother mid-range power. New spark plugs bring back that crisp, easy starting you remember from the first day you ran the engine.

The impeller replacement is one of the most important steps in the service. A strong, healthy water pump keeps temperatures stable so your engine stays protected even during long idles, extended trolling runs, or full-throttle stretches across open water. Fresh gearcase seals add another layer of defense by reducing the likelihood of water intrusion, helping gears and bearings last longer.

Every part in this kit works together to give you that tight, steady, trustworthy running quality Mercury FourStrokes are known for. When your maintenance is done right, you feel it every time you leave the dock.

Why Buy OEM

• Genuine Mercury parts ensure exact fit and consistent performance
• Designed specifically for your 25–30hp FourStroke—no guessing, no mismatches
• More durable and reliable than many aftermarket alternatives
• Helps preserve engine life and protect your investment
• Backed by decades of Mercury Marine engineering and testing

Seasoned boaters will tell you the same thing: OEM parts simply hold up better, especially when your engine earns its hours in real-world conditions.

FAQ

Q1: Which engines does the 8M0120839 kit fit?

It fits Mercury 25–30hp FourStroke outboards within serial numbers 0R106999–1R162636.

Q2: How is this different from aftermarket kits?

Mercury filters, seals, and components are engineered to tighter tolerances and built for long-term marine durability.

Q3: How do I confirm this is the right kit for my outboard?

Check your engine’s serial number and match it to the supported range.

Q4: Can this kit be used in saltwater and freshwater?

Yes—every part is designed to handle both environments.

Q5: Is the 300-hour service manageable for most boat owners?

If you’re comfortable with routine maintenance, yes. Otherwise, any Mercury dealer can perform the service.