Lower Unit Seal Kit | Sierra 18-2783

Sierra 18-2783 OMC King Cobra Lower Gearcase Seal Kit | 1990 | Replaces OMC 986629

The Sierra #18-2783 is a lower gearcase seal kit for OMC King Cobra sterndrive lower gearcases from 1990. Replaces OMC 986629. Confirm your King Cobra drive is a 1990 model year before ordering — King Cobra gearcase seal kits are year-specific.

Part Details:

• Part Number: 18-2783 (Sierra)
• SKU: SIERRA-18-2783
• Manufacturer: Sierra
• Type: Lower Gearcase Seal Kit
• Fits: OMC King Cobra lower gearcase, 1990
• Replaces: OMC 986629
• Condition: New

Cross-Reference — Replaces:

• OMC: 986629

Additional Information

OMC King Cobra — Year-Specific Seal Kits:

The OMC King Cobra sterndrive used different lower gearcase seal kits across its production years. The Sierra 18-2783 is specific to 1990 King Cobra lower gearcases. For 1992–1995 King Cobra drives, see the Sierra 18-2791. Confirm your King Cobra drive's model year from the drive identification plate before ordering — using the wrong year's seal kit risks incorrect seal sizing and immediate seal failure.

Lower Gearcase Seal Kit — What It Seals:

The lower gearcase seal kit contains the seals and O-rings that prevent water from entering the King Cobra lower gearcase and gear lube from leaking out. Primary sealing points include:

• Prop shaft seal(s) — dual lip seals at the propeller shaft exit; the most common King Cobra seal failure point
• Input shaft seal — seals the drive shaft where it enters the lower gearcase from the upper unit
• Shift shaft seal — seals the shift rod at the lower gearcase housing
• Housing O-rings and gaskets — seal gearcase mating surfaces and drain/fill ports

Gearcase Seal Failure — Symptoms:

Lower gearcase seal failure allows water into the gear lube or allows gear lube to leak out. Signs include:

• Milky, cream-colored, or frothy gear lube when drained — the definitive indicator of water intrusion
• Gear lube level dropping between service intervals
• Oil film or lube discharge around the prop shaft area or at gearcase vents

Water-contaminated gear lube causes accelerated gear and bearing wear. Replace seals immediately at the first sign of water contamination and inspect gears and bearings before refilling.

Prop Shaft Seal Orientation — Critical:

Prop shaft lip seals are installed back-to-back in opposing directions — the outer seal lip faces outward to exclude water; the inner seal lip faces inward to retain gear lube. Installing lip seals in the wrong orientation causes immediate seal failure after reassembly. Refer to the OMC King Cobra service manual for the correct seal orientation diagram for the 1990 gearcase before installation.

Inspect During Service:

When resealing the lower gearcase, inspect simultaneously:

• Prop shaft for scoring or grooving at the seal contact surface — a worn shaft will destroy new seals
• Input shaft for wear at the seal contact surface
• Gearcase housing bores for corrosion or elongation
• Gear lube condition — if milky, inspect gears and bearings before reassembly
• Water pump impeller — replace if not recently serviced while the drive is partially disassembled

Installation Notes:

• Confirm drive is OMC King Cobra 1990 before ordering — not for other King Cobra years or other OMC sterndrive models
• Install prop shaft lip seals in correct orientation per OMC King Cobra service manual
• Lubricate seal lips with gear lube before installation — do not install dry
• Use correct OMC sterndrive gear lube specification for refill
• Pressure and vacuum test the lower gearcase after reassembly before returning to service

Frequently Asked Questions

1. What year King Cobra does this seal kit fit?
1990 only. King Cobra gearcase seal kits are year-specific — confirm the model year from your drive identification plate before ordering. For 1992–1995 King Cobra drives, see the Sierra 18-2791.

2. What OMC part number does this replace?
OMC 986629.

3. What does milky gear lube mean?
Water has entered the gearcase through a failed seal. Drain immediately, identify and replace the failed seal(s), and inspect gears and bearings for corrosion or damage before refilling with fresh gear lube.

4. Why is prop shaft seal orientation critical?
Lip seals installed in the wrong direction will not seal — water enters or gear lube exits immediately after reassembly. Refer to the OMC King Cobra service manual for the 1990 gearcase seal orientation diagram before installation.

5. Should I replace the water pump impeller at the same time?
Yes, if not recently serviced. The lower unit is partially disassembled during gearcase seal service — it is efficient to replace the water pump impeller at the same time. Annual impeller service is recommended for all King Cobra sterndrive units.