Lower Unit Drain Plug Kit – Fits Mercury/MerCruiser Outboard Engines & Sterndrives

Lower Unit Drain Plug Kit – Fits Mercury/MerCruiser Outboard Engines & Sterndrives (Replaces 8M0058389, 79953Q04)

This lower unit drain/fill plug kit is designed for a wide range of Mercury and Mariner outboard engines, as well as MerCruiser sterndrives. It fits most models equipped with 3/8-16 drain and fill screw holes, excluding the Verado series. Compatible sterndrive models include MC-1, Alpha One (Gen I & II), TR, TRS, I, R, and MR.

This kit serves as a direct replacement for multiple OEM and aftermarket part numbers, including Mercury/Quicksilver 8M0058389 (magnetic) and 79953Q04 / 10-79953Q04 (non-magnetic).

• Magnetic Protection: The primary drain plug features a strong magnetic tip designed to capture fine metallic particles within the gear lube, preventing wear and protecting internal components.
• Complete Kit: Each kit includes one drain/fill screw and a new gasket/seal to ensure a leak-free installation.
• Threaded Installation: Designed for a secure fit with a standard 3/8-16 UNC thread size.

Part Number Cross-Reference

Use this table to confirm equivalent part numbers across OEM and aftermarket brands.

Compatible Engine Models

Confirm your engine model/year in the table below. For Mercury owners, here’s a guide to help you identify your engine’s serial/model: Mercury Serial Number Guide. You can also review exploded diagrams to verify part numbers: Mercury Parts Diagrams & Catalog.

MerCruiser Models

Mercury & Mariner Models

Technical Specifications

Package Contents

This product is available in multiple configurations. Please verify the contents of the specific kit you are purchasing.

• Magnetic Version (Replaces 8M0058389):
  • 1 x Magnetic Drain Screw (8M0058389)
  • 1 x Gasket/Seal
• Non-Magnetic Version (Replaces 79953Q04):
  • 1 x Non-Magnetic Drain Screw (79953Q04)
  • 1 x Gasket/Seal

Note: Some kits may include both a magnetic and a non-magnetic screw along with two gaskets.

Installation Guide

1. Prepare Your Workspace: Position a suitable oil drain pan under the lower unit drain plug. The plug is typically the lowest screw on the gearcase.
2. Remove Plugs: Loosen and remove both the lower drain plug and the upper vent plug. Removing the vent plug allows oil to flow out smoothly.
3. Drain Fluid: Allow the gear lube to drain completely. This may take several minutes. Inspect the old oil for any signs of water intrusion (milky appearance) or excessive metal shavings.
4. Clean Threads: Wipe the threads around the drain and vent holes on the gearcase to ensure they are clean and free of debris.
5. Install New Drain Plug: Install the new magnetic drain plug with its new gasket into the lower drain hole. Tighten it securely with a screwdriver, but do not overtighten.
6. Refill Gear Lube: Insert the nozzle of your gear lube pump into the lower drain hole and pump in fresh lubricant until it begins to flow out of the upper vent hole.
7. Install Vent Plug: While keeping the lube tube in the drain hole, install the upper vent plug with its gasket and tighten it securely. This prevents lube from draining out.
8. Finalize Installation: Quickly remove the lube tube from the drain hole and reinstall the drain plug. Tighten it firmly.
9. Clean and Inspect: Wipe away any excess oil and inspect both plugs for leaks after a few minutes.

Troubleshooting & FAQ

How do I confirm this plug kit fits my engine?
Start by checking the compatibility tables on this page. The most reliable method is to cross-reference the part number stamped on your old drain screw with the numbers listed in our "Part Number Cross-Reference" table. This kit fits most Mercury, Mariner, and MerCruiser models with 3/8-16 threaded drain holes.

What is the purpose of the magnetic plug?
The magnetic drain plug (8M0058389) is designed to attract and capture any microscopic ferrous metal particles circulating in your gear lube. This helps monitor the health of your lower unit and prevents those particles from causing premature wear on your gears and bearings.

What is the difference between the magnetic (8M0058389) and non-magnetic (79953Q04) versions?
Both plugs serve the same function of sealing the drain/fill hole. The 8M0058389 includes a magnet for added gear protection, while the 79953Q04 is a standard, non-magnetic screw. Both use the same gasket.

How often should I replace the drain plug gasket?
The gasket should be replaced every time you change your lower unit gear lube. This ensures a proper, leak-proof seal. Our kits include a new gasket for this purpose.

OEM vs. Aftermarket?
This listing is an aftermarket replacement part (not an OEM component). We build to OEM specifications using comparable-grade materials. Production is done in an ISO9002-certified facility, and each batch is inspected for consistent performance.

How fast do you ship and how much does it cost?
We offer fast global shipping—typically 5–15 days worldwide—and standard shipping is free. Expedited options are available at checkout depending on your location.

What is your warranty and return policy?
This product includes a 1-year warranty against manufacturing defects under normal use. If something goes wrong, contact our support team with your order details. Returns are accepted within our standard return window (unused & in original condition); please see our policy page or reach out for instructions.

Important Notes

• Verify that the product images and specifications match your original part. If discrepancies exist—even if your model is listed—do not proceed with the purchase.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.

Why Choose JLM MARINE?

• Quality Assured: As a trusted manufacturer for over 20 years, we supply top marine brands globally.
• Fast Global Shipping: Free delivery worldwide in 5–15 days.
• Direct from Manufacturer: Competitive pricing without compromising quality.