Mercury – Lower Unit Gear Lube Drain and Fill Hole Screw & Seal, Magnetic | Gearcase Hardware | Magnetic Steel | 8M0058389

Overview

There’s a certain kind of comfort that comes from knowing the simplest parts on your boat are the ones you never need to think about. Anyone who’s been around the water long enough knows it’s often the little things — a seal, a plug, a gasket — that keep a good day from turning into a long tow back to the dock. That’s why a dependable lower unit drain and fill screw matters more than most people realize. With the Mercury Lower Unit Gear Lube Drain and Fill Hole Screw & Seal (Magnetic), you get that quiet reassurance that your gearcase is buttoned up tight, protected from leaks, and shielded from the damage water intrusion can cause.

Key Features

• Designed for use with 3/8-16 drain and fill hole screws
• Fits all MerCruiser sterndrives, Mercury outboards, and Mariner outboards except Verado models
• Includes the required seal to ensure proper seating and leak prevention
• Magnetic plug attracts ferrous metal shavings to keep them out of circulation
• Helps prevent gear lube loss and protects the gearcase from water intrusion
• Built to Mercury Marine OEM specifications for durability and reliability

Engine & Boat Compatibility

• Compatible with most Mercury and Mariner outboards (non-Verado)
• Works with many MerCruiser sterndrive models
• Commonly used on fishing boats, pontoons, center consoles, runabouts, cruisers, and sport boats
• Fits gearcases requiring a 3/8-16 drain and fill screw

Performance & Practical Benefits

The performance of this small but essential part becomes obvious the moment you service your lower unit. The screw threads in smoothly, seats firmly, and gives you that satisfying snug fit that tells you the seal is doing its job. Once installed, it holds the gear lube securely inside the case, keeping the system running cool, lubricated, and protected.

The magnetic feature is what sets this version apart. As the gears work, tiny metal particles naturally shed — nothing alarming, but enough that you don’t want them floating around in the oil. This screw’s built-in magnet draws those particles in and holds them there. That means cleaner gear lube, smoother operation, and less long-term wear on the gears and bearings. It’s a small detail that quietly adds years of life to a lower unit.

What you’ll feel on the water is simple: a smoother shift, a steady hum, and the peace of mind that comes from knowing the gearcase is sealed tight against water intrusion. Saltwater or freshwater, it keeps the lower unit protected and ready for another season of hard use.

Why Buy OEM

A genuine Mercury Marine part isn’t just about brand loyalty — it’s about getting a component designed to fit perfectly and last. Aftermarket screws often use cheaper materials, inconsistent threads, or weaker magnets, which can lead to leaks, stripped housings, or gear wear. OEM means proper fitment, exact tolerances, and engineering built around your specific gearcase. When the health of your lower unit depends on a few drops of gear lube staying where they belong, cutting corners isn’t worth it. This is the practical, proven choice.

FAQ

Q1: Which Mercury engines does this screw fit?

It fits most Mercury and Mariner outboards and MerCruiser sterndrives, excluding Verado models.

Q2: How is it different from aftermarket drain screws?

OEM screws offer better sealing, stronger magnets, and precise fitment that generic versions rarely match.

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

If your gearcase uses a 3/8-16 drain and fill screw and is not a Verado, this is the correct part.

Q4: Can it be used in saltwater and freshwater?

Absolutely. It’s built for both environments and resists corrosion when properly maintained.

Q5: Is installation straightforward?

Yes — remove the old screw, replace the seal, fill or drain the gear lube as needed, and install this screw snugly.