26-77066A1 Gearcase Seal Kit for Mercury Mariner Force | JLM Marine

Gearcase Seal Kit – Fits Mercury, Mariner, and Force 3.9–9.8 HP Outboards (1975–1986)

This gearcase seal kit is designed to fit 3.9–9.8 HP Mercury, Mariner, and Force outboard lower units manufactured from 1975 to 1986. It provides a complete sealing solution for your lower unit, ensuring reliable protection against water intrusion and gear oil leaks.

Direct replacements: Mercury 26-77066A1, Sierra 18-2628. This kit is engineered to meet OEM specifications and delivers dependable performance for vintage outboard motors.

• Complete seal kit with all necessary components for lower unit maintenance
• Compatible with multiple Mercury, Mariner, and Force models from 1975–1986
• Aftermarket replacement built to OEM standards

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

This gearcase seal kit includes:

• 1× Complete Gearcase Seal Kit (The package contains all the parts shown in the product image.)

Installation Guide

1. Preparation: Disconnect the spark plug wire and drain all gear oil from the lower unit. Clean the work area thoroughly.
2. Remove Lower Unit: Detach the lower unit from the mid-section by removing the mounting bolts and shift linkage.
3. Disassemble Gearcase: Remove the propeller, propeller shaft, and drive shaft. Take care to note the orientation of components for reassembly.
4. Remove Old Seals: Carefully pry out old seals using a seal puller or flat-blade screwdriver. Avoid damaging the seal housing surfaces.
5. Clean Housing Surfaces: Thoroughly clean all seal housing areas with solvent and a lint-free cloth. Inspect for wear, corrosion, or damage.
6. Install New Seals: Lightly lubricate new seals with gear oil. Press seals into place squarely using a seal driver or socket of appropriate diameter.
7. Reassemble Gearcase: Reinstall drive shaft, propeller shaft, and related components in reverse order of disassembly. Ensure all components are properly aligned.
8. Reattach Lower Unit: Mate the lower unit to the mid-section, reconnect shift linkage, and torque mounting bolts to manufacturer specifications.
9. Refill Gear Oil: Fill the lower unit with the recommended gear oil through the lower fill hole until oil appears at the upper vent hole. Install and tighten both plugs.
10. Final Check: Reinstall the propeller, reconnect the spark plug wire, and test-run the engine in a test tank or on the water. Check for leaks and proper shifting.

Troubleshooting & FAQ

Q: How do I confirm this seal kit fits my engine?
A: Cross-reference your engine's HP rating (3.9–9.8 HP) and manufacturing year (1975–1986) with the compatibility table above. Verify your original part number matches Mercury 26-77066A1 or Sierra 18-2628.

Q: What are the signs that my gearcase seals need replacement?
A: Common symptoms include visible gear oil leaks around the lower unit, water contamination in the gear oil (milky appearance), difficulty shifting gears, or unusual noise from the lower unit.

Q: How often should I replace the gearcase seals?
A: Replace seals whenever you notice leaks, water intrusion, or during major lower unit service (typically every 3–5 years or as needed).

Q: Can I install this seal kit myself?
A: Installation requires disassembly of the lower unit and familiarity with outboard service procedures. If you have mechanical experience and the proper tools, DIY installation is possible. Otherwise, professional installation is recommended.

Important Notes

• Verify that the product images and specifications match your original part before purchasing.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.