Lower Unit Drain Plug Gearcase Screw – Fits Mercury & MerCruiser Outboards and Sterndrives (Replaces 10-79953Q04, 18-2244)

Lower Unit Drain Plug Gearcase Screw – Fits Mercury & MerCruiser Outboards and Sterndrives (Replaces 10-79953Q04, 18-2244)

This lower unit drain and fill plug screw is designed for Mercury and Mariner outboards equipped with a 3/8-16 drain hole, and is compatible with all MerCruiser sterndrives except for the Verado series. It serves as a direct, non-magnetic replacement for various OEM parts, ensuring a secure seal to prevent gear lube leakage and water intrusion.

Engineered from non-magnetic stainless steel, this drain plug provides reliable performance and corrosion resistance for marine environments. Each screw comes pre-fitted with a seal to ensure a leak-proof installation.

• Material: Non-magnetic stainless steel for durability and corrosion resistance.
• Installation: Requires a standard flat blade screwdriver.
• Package Includes: Two drain plug screws with pre-installed seals.

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.

Technical Specifications

Package Contents

This package includes the following items:

• 2 x Lower Unit Drain Plug Gearcase Screw with Seal (Replaces 10-79953Q04)

Installation Guide

1. Position a suitable drain pan under the lower unit's gearcase.
2. Using a large flat blade screwdriver, carefully loosen and remove both the upper (vent) and lower (drain) plugs.
3. Allow all the old gear lube to drain completely. Inspect the lube for any signs of metal shavings or water (milky appearance).
4. Clean the threads of the drain and vent holes on the gearcase.
5. Ensure the new drain plugs have their seals correctly seated.
6. Install one new plug into the upper vent hole, but do not tighten it fully.
7. Pump new gear lube into the lower drain hole until it begins to flow out of the upper vent hole.
8. Install the second new plug into the lower drain hole and tighten it securely.
9. Tighten the upper vent plug. Do not overtighten either plug; a snug fit is sufficient.
10. Wipe the gearcase clean of any excess lube.

Troubleshooting & FAQ

How do I confirm this fits my engine?
First, check the "Compatible Engine Models" table. This plug fits most Mercury, MerCruiser, and Mariner outboards and sterndrives that use a 3/8-16 thread screw, but it notably excludes Verado series models. Cross-referencing your old part number with our "Part Number Cross-Reference" table is the most reliable method.

Is this drain plug magnetic?
No. This is a non-magnetic version made from high-grade stainless steel. Magnetic plugs are designed to capture metallic debris from the gear lube, while non-magnetic plugs are often preferred in certain applications to avoid interfering with electronic sensors.

How often should I replace the drain plug and its seal?
The seal (gasket) should be inspected every time you change the gear lube and replaced if it appears cracked, hardened, or deformed to ensure a proper seal. The screw itself only needs replacement if the threads are stripped, the head is damaged, or if it is otherwise compromised.

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.