Magnetic Lower Unit Drain Plug with Gasket – Fits Mercury/MerCruiser (Replaces 8M0058389, 79953Q04)
Magnetic Lower Unit Drain Plug with Gasket – Fits Mercury/MerCruiser (Replaces 8M0058389, 79953Q04) is backordered and will ship as soon as it is back in stock.
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Magnetic Lower Unit Drain Plug with Gasket – Fits Mercury/MerCruise...
Magnetic Lower Unit Drain Plug with Gasket – Fits Mercury/MerCruiser (Replaces 8M0058389, 79953Q04) is backordered and will ship as soon as it is back in stock.
Product Details
This Sea Sierra magnetic lower unit drain plug is designed for compatible Mercury and MerCruiser gearcases that use the corresponding threaded drain and fill screw. The magnetic tip helps collect ferrous particles suspended in the gear lubricant, making the plug useful during routine gear-lube service and inspection.
The replacement plug is listed for interchange with several Mercury/Quicksilver and aftermarket references, including Mercury/Quicksilver 8M0058389 and 79953Q04, Sierra 18-2375, and GLM 21731. Always compare the thread, head style, sealing surface, and original part number before installation.
- Magnetic drain-plug design helps collect ferrous metal particles from the gearcase lubricant.
- Threaded installation is intended for compatible lower-unit drain and fill openings.
- Supplied with a sealing gasket for replacement during gear-lube service.
- Aftermarket replacement part; not manufactured by Mercury Marine.
Part Number Matrix
The references below are listed as replacement or interchange numbers. They should be used together with a physical-part and application check; the numbers do not by themselves confirm fitment for every engine or sterndrive.
| Brand | Part Number | Reference Type |
|---|---|---|
| Mercury / Quicksilver | 8M0058389 | OEM reference |
| Mercury / Quicksilver | 79953Q04 | OEM reference |
| Mercury | 67892A1 | OEM reference |
| Mercury | 67892A05 | OEM reference |
| Mercury | 67892A06 | OEM reference |
| Mercury | 14834A1 | OEM reference |
| Sierra | 18-2375 | Aftermarket reference; magnetic drain plug |
| Sierra | 18-23751-9 | Related package reference |
| GLM | 21731 | Aftermarket reference |
| Mallory | 9-72651 | Aftermarket reference |
Application and Compatibility
This plug is intended for compatible Mercury and Mariner outboard lower units and MerCruiser sterndrives using the matching drain/fill plug specification. Commonly referenced MerCruiser drive families include MC-1, Alpha One, Alpha One Gen II, TR, TRS, I, R, and MR, but application can vary by year, gearcase, serial number, and production change.
Do not rely on the engine brand or model family alone. Confirm the original plug, thread size, sealing surface, and manufacturer part number before ordering. Verado and other applications using a different drain-plug design may not be compatible.
| Application Area | Use | Verification Required |
|---|---|---|
| Mercury and Mariner outboards | Lower-unit gear-lube drain or fill opening | Confirm exact model, year, serial range, and original plug |
| MerCruiser sterndrives | Drive or lower-unit gear-lube drain/fill opening | Confirm drive family and plug specification |
| Verado and other specialty applications | Application may use a different plug design | Do not install without exact part-number confirmation |
Technical Specifications
- Product Type
- Magnetic lower-unit drain plug with sealing gasket
- Installation
- Threaded replacement installation
- Thread Size
- 3/8-16 UNC, where specified by the matching application
- Plug Function
- Lower-unit gear-lube drain/fill closure
- Magnetic Element
- Yes; intended to collect ferrous particles in the lubricant
- Finish
- Metallic silver/chrome appearance
- Seal
- Replacement sealing gasket included
- Construction
- Metal threaded plug with magnetic tip
What's in the Box
- 1 x Magnetic lower-unit drain plug
- 1 x Sealing gasket
This listing is for the magnetic plug configuration. It does not include a gear-lube pump, gear lubricant, additional vent plug, or installation tools.
Installation Notes
- Place a suitable drain pan below the outboard lower unit or sterndrive.
- Remove the upper vent screw and lower drain screw with the correct large flat-blade driver.
- Allow the old gear lubricant to drain completely. Inspect it for excessive metal particles or a milky appearance, which may indicate internal wear or water intrusion.
- Clean the area around the drain and vent openings and remove the old gasket material.
- Fit the new gasket to the magnetic drain plug and install it in the correct lower opening by hand to avoid cross-threading.
- Fill the gearcase with the lubricant specified by the engine or sterndrive manufacturer through the lower opening until lubricant appears at the upper vent opening.
- Install the upper vent screw with the appropriate new gasket if required by the application.
- Remove the lubricant pump and promptly install the lower drain plug with its gasket.
- Tighten both screws to the torque specified in the applicable service manual. Do not overtighten.
- Clean excess lubricant and check both openings for leaks after service.
Troubleshooting & FAQ
Will this plug fit every Mercury or Mariner outboard?
No. Mercury and Mariner applications vary by model, year, serial range, gearcase, and production change. Confirm the original plug and exact reference number before ordering.
Will it fit a Verado outboard?
Do not assume compatibility with Verado models. Some applications use a different drain-plug design. Confirm the exact service-part requirement for your engine before installation.
What does the magnet do?
The magnetic tip is intended to collect ferrous particles in the gear lubricant. Inspecting the plug during an oil change can help identify abnormal wear, although the plug is not a substitute for a complete mechanical inspection.
Should I replace the gasket?
Yes. Replace the sealing gasket whenever the drain plug is removed if the application uses a replaceable gasket. A new gasket helps reduce the risk of seepage.
What torque specification should I use?
Torque specifications vary by engine and sterndrive model. Use the applicable manufacturer service manual rather than relying on a general tightening rule. Avoid overtightening, which can damage the threads, gearcase, plug, or sealing surface.
Is this an OEM Mercury part?
No. This is an aftermarket replacement that references the listed Mercury/Quicksilver part numbers for application comparison.
Safety & Important Information
- Verify the original part number, thread, gasket style, and physical dimensions before purchase.
- Keep the model, year, horsepower, stroke, shaft, and serial-number conditions together when checking compatibility.
- Use only the gear lubricant specified for your engine or sterndrive.
- Dispose of used gear lubricant according to local regulations.
- Professional installation is recommended if you are unsure of the correct drain and vent openings or torque specification.
About Sea Sierra
- Marine replacement parts developed for routine service and repair applications.
- Product references are provided to help compare compatible replacement components.
- Confirm application details before installation for the best fit and sealing performance.
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