Gearcase Drain Plug – Fits Mercury/MerCruiser Marine Engines (Replaces 10-79953Q04, 79953Q04)

Gearcase Drain Plug – Fits Mercury/MerCruiser Marine Engines (Replaces 10-79953Q04, 79953Q04)

This gearcase drain and fill plug is designed for a wide range of Mercury outboard engines, including 30-125 HP EFI models, and various MerCruiser sterndrive units. It serves as a crucial component for draining and refilling gearcase lubricant in the lower unit.

This part is a direct replacement for Mercury part numbers 10-79953Q04, 79953Q04, 10-79953A2, 10-79953Q2 and Sierra part number 18-2244.

• Installation Method: Simple threaded screw-in installation.
• Material: Constructed from durable brass for corrosion resistance in marine environments.
• Complete Assembly: Includes the required sealing washer/gasket for a leak-proof fit.

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

• 1 x Lower Unit Gear Lube Drain w/Fill Hole Screw (79953Q04)
• 1 x Seal/Gasket (included with assembly)

Installation Guide

1. Locate the gearcase drain/fill plugs on the lower unit. There are typically two: an upper (vent) and a lower (drain).
2. Position a suitable drain pan underneath the lower plug.
3. Remove the lower drain plug using a large flathead screwdriver, allowing the old gear lube to drain completely.
4. Remove the upper vent plug to allow air to enter, which speeds up the draining process.
5. Inspect the old plugs and magnetic tips for any metal shavings, which can indicate excessive gear wear.
6. Clean the threads on the gearcase housing before installation.
7. Install the new drain plug with its gasket in the lower hole and tighten it securely, but do not overtighten.
8. Fill the gearcase with the manufacturer-specified lubricant through the upper vent hole until fluid begins to flow out.
9. Install the new upper vent plug with its gasket and tighten securely.
10. Clean any excess lubricant from the lower unit housing.

Troubleshooting & FAQ

How do I confirm this fits my engine?
First, check your existing part for a part number and compare it to those listed in our cross-reference table. Second, review the compatible engine models table. For Mercury engines, use the provided Mercury Serial Number Guide to confirm your model details.

Is this used for both draining and filling the gearcase?
Yes, this screw and seal assembly is used for both the lower drain hole and the upper vent/fill hole on compatible Mercury and MerCruiser lower units.

What is the purpose of the included gasket?
The gasket (seal) is critical for preventing gear lube from leaking out and water from entering the gearcase. Always use a new gasket with each installation to ensure a proper seal.

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.