OMC Lower Gearcase Seal Kit | Sierra 18-2791

Sierra 18-2791 OMC Lower Gearcase Seal Kit | 3.0L–8.2L 1992–1995 | Replaces OMC 987251, 3850597 | Volvo Penta 3850597

The Sierra #18-2791 is a lower gearcase seal kit for OMC sterndrive units. Covers OMC 3.0L, 4.3L, 5.0L (1994), 3.2L (1992–1993), 5.7L (1992–1994), 7.4L (1992–1995), and 8.2L (1992–1993). Replaces OMC 987251 and 3850597; Volvo Penta 3850597.

Part Details:

• Part Number: 18-2791 (Sierra)
• SKU: SIERRA-18-2791
• Manufacturer: Sierra
• Type: Lower Gearcase Seal Kit
• Condition: New

Fitment — OMC Sterndrive by Engine Displacement and Year:

• 3.0L: 1994
• 3.2L: 1992–1993
• 4.3L: 1994
• 5.0L: 1994
• 5.7L: 1992–1994
• 7.4L: 1992–1995
• 8.2L: 1992–1993

Cross-Reference — Replaces:

• OMC: 987251, 3850597
• Volvo Penta: 3850597

Additional Information

OMC and Volvo Penta — Shared Gearcase Design:

The shared part number (3850597) between OMC and Volvo Penta reflects the common lower gearcase design used across OMC Cobra/King Cobra and Volvo Penta SX and DP sterndrive units during this era. OMC and Volvo Penta jointly developed and shared sterndrive lower unit designs in the early 1990s, resulting in interchangeable seal kits across both brands for the same gearcase configuration. The Sierra 18-2791 serves both OMC and Volvo Penta applications that reference part number 3850597.

Lower Gearcase Seal Kit — What It Seals:

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

• Prop shaft seal(s) — dual lip seals at the propeller shaft exit; the most common sterndrive 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 housing mating surfaces and drain/fill ports

Gearcase Seal Failure — Symptoms:

Lower gearcase seal failure causes water intrusion into the gear lube or gear lube loss. Symptoms include:

• Milky, cream-colored, or frothy gear lube when drained — definitive sign of water intrusion
• Gear lube level dropping between service intervals
• Oil film or lube discharge visible around the prop shaft or at gearcase vents
• Water in the bilge with no other apparent source

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

Prop Shaft Seal Orientation — Critical:

Prop shaft lip seals are typically installed back-to-back (dual lip, opposing direction). The outer seal lip faces outward to exclude water; the inner seal lip faces inward to retain gear lube. Incorrect installation orientation causes immediate seal failure. Refer to the OMC service manual for your specific sterndrive model for the correct seal orientation diagram before installation.

Service at Every Seal Replacement:

When resealing the lower gearcase, inspect simultaneously:

• Prop shaft for scoring or grooving at the seal contact surface — a worn shaft destroys new seals
• Input shaft for wear at the seal contact surface
• Gearcase housing bores for wear, corrosion, or damage
• 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 engine displacement and year against fitment table before ordering
• Install prop shaft lip seals in correct orientation — refer to OMC service manual for seal direction diagram
• Lubricate seal lips with gear lube before installation — do not install dry
• Use the correct OMC or Volvo Penta sterndrive gear lube specification for refill
• Pressure and vacuum test the lower gearcase after reassembly before returning to service

Frequently Asked Questions

1. Does this kit also fit Volvo Penta sterndrive units?
Yes — the shared OMC/Volvo Penta part number 3850597 reflects a common lower gearcase design used across OMC Cobra/King Cobra and Volvo Penta SX and DP drives in this era. The Sierra 18-2791 replaces both OMC 3850597 and Volvo Penta 3850597.

2. What does milky gear lube mean?
Milky or cream-colored gear lube indicates 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.

3. What engine sizes does this kit fit?
OMC 3.0L and 4.3L and 5.0L (1994); 3.2L (1992–1993); 5.7L (1992–1994); 7.4L (1992–1995); 8.2L (1992–1993). Confirm your engine displacement and year before ordering.

4. Why is prop shaft seal orientation so important?
Prop shaft lip seals installed in the wrong direction will not seal — water enters or gear lube exits immediately after reassembly. Refer to the OMC service manual for your drive model for the correct back-to-back lip seal orientation 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, making it an efficient time to replace the water pump impeller. Annual impeller replacement is recommended for all OMC sterndrive units.