Gearcase Seal Kit – Fits Mercury/Mariner 3.9-9.8 HP Outboards (Replaces 26-77066A1)

$37.95

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Product Details

This aftermarket Gearcase Seal Kit provides the necessary components to reseal the lower unit on various Mercury, Mariner, and Force outboards. It is specifically designed for models ranging from 3.9 HP to 9.8 HP manufactured between 1975–1986.

This kit serves as a direct replacement for Mercury/Mariner OEM part number 26-77066A1, Sierra 18-2628, GLM 87574, and Mallory 9-74212.

Installation: Direct replacement for lower unit/gearcase.
Function: Replaces worn or damaged seals to prevent water intrusion and oil leaks in the gearcase.
Application: Water pump seal kit for lower unit/gearcase.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket.

Brand	Part Number
Mercury/Mariner	26-77066A1
Sierra	18-2628
GLM	87574
Mallory	9-74212

Engine & Model Compatibility

Confirm your engine model/year in the table below.

This kit is designed for select Mercury, Mariner, and Force models. Please check your serial number to ensure correct fitment.

Brand	Model	Rated HP	Years	OEM Part No.	Notes
Mercury	1 Cylinder	4		26-77066A1	S/N 5595532–A809600
Mercury	1 Cylinder	4.5		26-77066A1	S/N 4107220 & up
Mercury		7.5		26-77066A1	S/N 4851693 & up
Mercury		9.8		26-77066A1	S/N 4839254 & up
Mercury/Force		3.9	1975–1986	26-77066A1
Mercury/Force		6	1975–1986	26-77066A1
Mercury/Force		8	1975–1986	26-77066A1

Technical Specs

Product Type: Gearcase/Lower Unit Seal Kit
Installation: Direct replacement
Dimensions: 7.13" H x 6.0" W x 0.63" L
Warranty: Varies by manufacturer (typically 1 year)
Other Technical Attributes: Water pump seal kit for lower unit/gearcase

What’s in the Box

1 x Seal Kit, Water Pump (26-77066A1)

Installation Notes

Disconnect the battery and ensure the engine is stable and secure before beginning work.
Drain the gearcase lubricant by removing the drain and vent plugs. Inspect the old lube for signs of water (milky appearance).
Remove the propeller, thrust washer, and any associated hardware.
Unbolt and carefully separate the lower unit (gearcase) from the engine's drive shaft housing.
Disassemble the lower unit as required to access the old seals, noting the orientation and order of each component.
Thoroughly clean all sealing surfaces, removing any old sealant, corrosion, or debris.
Carefully install the new seals using appropriate seal drivers or sockets to avoid damage. Ensure they are fully and evenly seated.
Reassemble the lower unit, lubricating components as specified in your engine's service manual.
Reinstall the gearcase, ensuring the drive shaft, shift shaft, and water tube are correctly aligned.
Refill the gearcase with fresh, manufacturer-recommended gear lubricant to the correct level.
After running the engine (in water), check for any leaks around the seals and re-check the gear lube for any signs of water intrusion.

Troubleshooting & FAQ

How do I know if this seal kit fits my specific engine?

This kit is designed for Mercury, Mariner, and Force outboards from 3.9 HP to 9.8 HP, generally from 1975–1986. Refer to the Engine & Model Compatibility table and cross-reference your engine's model and serial number range for an exact match. Verifying against the OEM part number (26-77066A1) is the most reliable method.

What are the signs my gearcase seals are failing?

Common signs of failing gearcase seals include a milky or creamy appearance in the gear lube (indicating water has mixed with the oil), visible oil leaks around the propeller shaft or driveshaft housing, or an unusual amount of metal shavings on the magnetic drain plug.

How often should I replace my gearcase seals?

There is no strict replacement interval, but seals should be inspected for leaks every time you change your gear lube (at least annually). As a preventative measure, it is wise to replace them every 3-5 years, especially if the boat is used in saltwater, or immediately if any signs of failure appear.

Is this a complete kit for the entire lower unit?

This is a water pump seal kit for the lower unit/gearcase. While it contains the primary seals for that specific assembly, it may not include every single O-ring or seal for the entire gearcase. Please review the product images and contents list to ensure it meets your repair needs.

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

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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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