Lower Unit Drain Plug Kit – Fits Mercury/MerCruiser (Replaces 8M0058389, 79953Q04)
Lower Unit Drain Plug Kit – Fits Mercury/MerCruiser (Replaces 8M0058389, 79953Q04) is backordered and will ship as soon as it is back in stock.
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Worry-free returns
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Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Lower Unit Drain Plug Kit – Fits Mercury/MerCruiser (Replaces 8M005...
Product Details
This Sea Sierra Lower Unit Drain Plug Kit is designed for Mercury and MerCruiser lower units that utilize a 3/8-16 threaded drain/fill screw. It is broadly compatible with most Mercury & Mariner outboard models, excluding the Verado series, and fits a wide range of MerCruiser sterndrives, including MC-1, Alpha One (Gen I & II), TR, and TRS models.
The kit serves as a direct interchange for several part numbers, including Mercury/Quicksilver 8M0058389 and 79953Q04, Sierra 18-2375, and GLM 21731, ensuring a precise fit for your maintenance needs.
- The magnetic plug design captures ferrous metal particles, protecting internal gears from damage.
- Features a standard 3/8-16 UNC threaded connection for straightforward installation.
- Each kit includes a new gasket to ensure a leak-proof seal upon installation.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercury / Quicksilver | 8M0058389 |
| Mercury | 67892A1 |
| Mercury | 67892A06 |
| Mercury | 67892A05 |
| Mercury | 14834A1 |
| Mercury / Quicksilver | 79953Q04 |
| Mercury / Quicksilver | 10-79953Q04 |
| GLM | 21731 |
| Sierra | 18-2375 |
| Sierra | 18-23751-9 |
| Mallory | 9-72651 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
For MerCruiser models, this plug fits the lower unit gear lube drain/fill hole on the specified drive models.
| Brand | Model | OEM Part No. | Notes |
|---|---|---|---|
| MerCruiser | Alpha One | 8M0058389 | Lower unit gear lube drain/fill hole |
| MerCruiser | Alpha One Gen II | 8M0058389 | Lower unit gear lube drain/fill hole |
| MerCruiser | MC-1 | 8M0058389 | Lower unit gear lube drain/fill hole |
| MerCruiser | TR | 8M0058389 | Lower unit gear lube drain/fill hole |
| MerCruiser | TRS | 8M0058389 | Lower unit gear lube drain/fill hole |
| MerCruiser | I | 8M0058389 | Lower unit gear lube drain/fill hole |
| MerCruiser | R | 8M0058389 | Lower unit gear lube drain/fill hole |
| MerCruiser | MR | 8M0058389 | Lower unit gear lube drain/fill hole |
For Mercury & Mariner outboards, this plug fits various 2-stroke and 4-stroke models with 3/8-16 drain holes.
| Brand | Model | Rated HP | Stroke | OEM Part No. | Notes |
|---|---|---|---|---|---|
| Mercury | 3 Cyl EFI | 30-40 HP | 8M0058389 | As verified by JLM Marine | |
| Mercury | 4 Cyl | 35-60 HP | 8M0058389 | As verified by JLM Marine | |
| Mariner | 3.0L | 225-250 HP | 8M0058389 | Gear case assembly parts | |
| Mariner | Various | 35-70 HP | 2 & 4-Stroke | 8M0058389 | 2, 3 & 4-Cylinder models |
| Mariner | Various | 30-125 HP | 2 & 4-Stroke | 8M0058389 | 3 & 4-Cylinder, 4.25" (107.95mm) Big foot |
| Mariner | Various | 50-60 HP | 2 & 4-Stroke | 8M0058389 | 50, 55, 60 HP (3-Cyl 2-Stroke), 40, 50, 60 HP EFI (4-Cyl 4-Stroke) |
Technical Specs
- Product Type
- Lower Unit Drain Plug Kit
- Installation
- Threaded
- Connection
- 3/8-16 UNC thread
- Housing Material
- Metal with magnetic element
- Housing Color
- Chrome/Silver with yellow gasket
- Fitting Type
- Threaded plug with seal
- Thread Size
- 3/8-16 UNC
- Other Technical Attributes
- Magnetic plug design captures ferrous metal particles to prevent gear damage
What’s in the Box
This kit is available in multiple configurations. Please select your required version.
- For the magnetic version (8M0058389): 1 x Magnetic drain screw (8M0058389) and 1 x Gasket/seal.
- For the non-magnetic version (79953Q04): 1 x Non-magnetic drain screw (79953Q04) and 1 x Gasket/seal.
- Some sets may include both magnetic and non-magnetic screws with 2 gaskets.
Installation Notes
- Position a drain pan under the lower unit of your outboard or sterndrive.
- Remove the upper (vent) and lower (drain) screws using a large flathead screwdriver.
- Allow the old gear lube to drain completely. Inspect the old oil for metal shavings or a milky appearance, which can indicate water intrusion.
- Clean the area around the drain and vent holes.
- Install the new magnetic drain screw with its new gasket into the lower hole. Tighten until snug, but do not overtighten.
- Pump new gear lube into the lower hole until it begins to flow out of the upper vent hole.
- Re-install the original upper vent screw with a new gasket (not included in all kits).
- Quickly remove the gear lube pump and re-install the new lower drain plug. Tighten both plugs securely.
- Clean any excess oil and check for leaks after a short period.
Troubleshooting & FAQ
Will this plug fit my Verado outboard?
No, this drain plug kit is not compatible with Mercury Verado series outboards, which typically use a different plug design. Please verify your specific model's part number requirements.
What is the purpose of the magnet?
The magnetic tip on the drain screw is designed to capture any fine metallic particles or shavings that may be circulating in your gear lube. This helps prevent accelerated wear on your gears and bearings.
How often should I change the drain plug and gasket?
It is recommended to replace the drain plug gasket with every gear lube change to ensure a proper seal. The plug itself should be inspected for thread damage or loss of magnetism and replaced as needed.
What torque specification should I use to tighten the plug?
While exact torque specs can vary by engine model, a general rule is to tighten the plug until it is snug and then turn it an additional 1/8 to 1/4 turn. Overtightening can damage the gearcase or the plug itself. Always refer to your service manual if available.
Is the yellow gasket resistant to fuel and oil?
Yes, the included gasket is made from a high-quality material designed to be resistant to marine fuel, oil, and other environmental factors to maintain a durable, leak-free seal.
What's the difference between the magnetic and non-magnetic plug?
The magnetic plug (8M0058389) offers an extra layer of protection by trapping metal debris. The non-magnetic plug (79953Q04) functions as a standard seal. Both are OEM-specified for different applications or as supersessions, but the magnetic version is generally preferred for diagnostics and gear protection.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.
Shipping speed & cost
We ship worldwide quickly—typical transit 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
Warranty & returns
Covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window (unused & in original condition). See our policy page or reach out for instructions.
Safety & Important Info
- Verify images/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement, not an OEM component.
- Professional installation is recommended for safety and performance.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
- Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
- Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.
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