Yamaha Upper Gear Case Seal Replacement

Department: Lower Unit Repair | Level: Advanced | Est Cost: $150

Water intrusion in the lower unit is the leading cause of catastrophic gear failure. While most mechanics immediately blame the propeller shaft seals, the upper drive shaft seals located in the Bearing Carrier are frequently the actual failure point. These seals reside directly below the water pump, submerged in high pressure water during operation. This protocol details the extraction and rebuilding of the Upper Bearing Carrier.

Section 1: Required Tooling and Parts

The Upper Bearing Carrier is friction fit into the housing and locked in place by corrosion and O Ring compression. Removal requires specific extraction tooling.

Essential Service Tools

• Upper Bearing Carrier Puller (AMT0016): This specialized tool engages the carrier from underneath, allowing for vertical extraction without touching the delicate water pump mating surface.
• Seal Driver: A socket or PVC pipe of appropriate diameter to press new seals.
• Chisel or Punch: For removal of the old seized seals.
• 12mm Socket: For removal of the carrier retention bolts.

Consumables

• Upper Seal Kit: Contains two oil seals and one O Ring.
• Shift Shaft Seal: Small oil seal for the shift selector rod.
• Marine Grease: For O Ring lubrication.
• Scotchbrite Pad: For cleaning corrosion from the aluminum housing.

Section 2: Component Removal

1. Initial Disassembly

Remove the water pump housing to expose the Bearing Carrier. Remove the four 12mm bolts securing the carrier to the lower unit casing. These bolts are often seized; use manual torque to break them loose before utilizing power tools.

2. Carrier Extraction

Install the Upper Bearing Carrier Puller into the carrier housing. Adjust the jaws to grip the underside of the carrier lip. Tighten the lifting screw.

As you apply tension, the carrier will slide vertically out of the housing. Note the resistance caused by the salt build up on the O Ring.

3. Shift Shaft Cover Removal

Locate the small plastic pry points on the shift shaft cover. Gently work the cover loose. This component is fragile plastic; excessive force will fracture the housing. Remove the cover to access the shift seal.

Section 3: Seal Replacement Protocol

4. Removing Old Seals

Secure the carrier on a bench. Utilizing a chisel, tap the underside of the old oil seals to drive them out.
Warning: The needle bearing is located directly below the seals. Direct impact to the bearing cage will destroy the carrier assembly. Angle your chisel carefully to engage only the seal casing.

5. Surface Preparation

Clean the internal bore using a Scotchbrite pad and mineral spirits. The aluminum surface must be perfectly smooth. Run your fingertip around the bore; if you feel roughness, the new seals will not seat correctly.

6. Installing New Drive Shaft Seals

Yamaha utilizes a dual seal system. Orientation is critical for operation.

• Seal 1 (Lower): Press into the bore until fully seated.
• Seal 2 (Upper): Press on top of the first seal.
• Orientation: The spring side of the seal must face UP (towards the water pump). This orientation prevents high pressure water from forcing its way past the lip and into the oil reservoir.

7. Installing Shift Seals

Press the old seal out of the shift cover. Press the new seal in until it bottoms out. Install a new O Ring on the underside of the cover.

Section 4: Reassembly

8. Carrier Installation

Apply a light coating of Marine Grease to the new Carrier O Ring. Align the carrier with the bolt holes in the lower unit housing.

The fresh O Ring will create significant resistance. Use a slide hammer or a soft mallet to tap the carrier down until the bolt holes line up flush. Do not pull the carrier down using the bolts; this risks stripping the aluminum threads in the casing.

9. Water Pump Reinstallation

With the carrier seated, you may now proceed with the installation of the water pump kit. Ensure the keyway on the drive shaft is aligned with the impeller.

Conclusion

If you discover milky oil in your gearcase, do not assume it is the prop shaft seal. The upper carrier seals are subjected to higher heat and pressure. Replacing them using the correct extraction tools ensures a leak free lower unit for years to come.

For more lower unit repair tools, browse the Marine Tech Lower Unit Section.