Gearcase Drain Plug – Fits Mercury/MerCruiser Marine Engines (Replaces 10-79953Q04, 10-79953A2)

Gearcase Drain Plug – Fits Mercury/MerCruiser Marine Engines (Replaces 10-79953Q04, 10-79953A2)

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.

Directly replaces Mercury parts 10-79953Q04, 79953Q04, 10-79953A2, and 10-79953Q2, along with aftermarket parts Sierra 18-2244 and Mallory 9-72650.

• Designed for draining and refilling gear oil in Mercury/MerCruiser lower units.
• Constructed from non-magnetic brass with a 3/8-16 thread size.
• Includes the required sealing washer/gasket for a leak-proof installation.

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 Plug (79953Q04)
• 1 x Seal/Gasket (included with assembly)

Installation Guide

1. Position a suitable drain pan underneath the lower unit's gearcase.
2. Using a large flathead screwdriver, locate and remove both the upper (vent) and lower (drain) gear lube plugs.
3. Allow all of the old gear oil to drain completely. Observe the oil's color—a milky appearance suggests water has entered the gearcase, which requires further inspection.
4. Clean the area around the plug holes to prevent contaminants from entering the gearcase.
5. Insert the nozzle of your gear lube pump or tube into the lower drain hole.
6. Carefully pump fresh gear lube into the unit. Continue until lubricant begins to flow out from the upper vent hole, indicating the gearcase is full.
7. Install the new upper vent plug first, ensuring the gasket is in place. Tighten it securely but do not over-torque.
8. Quickly remove the lube tube from the bottom hole and install the new lower drain plug, again making sure its gasket is seated properly. Tighten it securely.
9. Wipe away any excess gear lube from the housing.
10. Properly dispose of the used gear oil in accordance with local environmental regulations.

Troubleshooting & FAQ

Q: How do I know if this plug fits my Mercury engine?

A: This plug is designed for a broad range of Mercury outboards and MerCruiser sterndrives. The best way to confirm fitment is to check your model in the "Compatible Engine Models" table above or match the OEM part numbers (e.g., 10-79953Q04, 10-79953A2). You can also verify by confirming your existing plug uses a 3/8-16 thread.

Q: Why is this plug described as non-magnetic?

A: Original OEM drain plugs are often magnetic to capture fine metal shavings from the gear oil, which can help diagnose internal gear wear. This aftermarket plug is made from non-magnetic brass and functions perfectly to seal the gearcase, but it does not have the diagnostic magnetic feature.

Q: Should I replace the gasket with every gear oil change?

A: Yes. It is strongly recommended to replace the drain plug seal (gasket) every time the gear lube is changed. This ensures a proper, leak-free seal and prevents water intrusion. This plug assembly includes a new, pre-installed seal.

Q: OEM vs. Aftermarket?

A: 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.

Q: How fast do you ship and how much does it cost?

A: 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.

Q: What is your warranty and return policy?

A: 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.