SeaSierra Outboard Gearcase Seal Kit for Johnson Evinrude 75–300HP (5006373) — Complete lower-unit seals & gaskets

The SeaSierra 5006373 gearcase seal kit is a complete lower-unit seal and gasket set engineered to restore water-tight performance for Johnson and Evinrude 75–300HP outboards with mechanical shift gearcases.

If your outboard lower unit leaks oil, draws water into the gearcase, or needs routine overhaul, this SeaSierra kit gathers the common oil seals, O-rings, gaskets, bearings and service hardware you’ll need to rebuild V4, V6 and V8 engines made from 1985–2005. It replaces multiple OEM part numbers so you can service a wide range of Johnson and Evinrude models without hunting individual components.

Which engines and years will this kit fit?

This kit is designed for Johnson and Evinrude outboards rated from 75HP up to 300HP with mechanical shift lower units. It supports V4, V6 and V8 configurations produced between 1985 and 2005, covering many common mid- and high-horsepower models.

• Covers 75–300HP models including 80, 85, 90, 100–175, 185–300HP ranges.
• Compatible with mechanical-shift gearcases on V4/V6/V8 engines.
• Works as a direct replacement for OEM numbers like 5006373 and several associated seals.

What components are included to complete a lower-unit service?

The SeaSierra 5006373 kit bundles the typical sealing parts and key small hardware used in a lower-unit overhaul so you won’t need to source many items separately. It focuses on oil seals, O-rings, gaskets, and bearing housing parts to restore proper sealing and drainage.

• Multiple oil seals and upper driveshaft seal for shaft and gearcase openings.
• O-rings and bearing-housing O-ring to renew internal sealing surfaces.
• Water pump gasket, shift cover gasket, fill/drain screws, washers, and bearing housing parts.

How does installing this kit help my boat’s performance?

Replacing degraded seals and gaskets prevents oil loss and water intrusion into the gearcase, which protects gears and bearings from corrosion and wear. Fresh seals also maintain correct lubrication and pressure inside the lower unit for reliable shifting and quieter operation.

• Prevents gearbox contamination by keeping oil in and water out.
• Reduces risk of bearing and gear damage from water intrusion.
• Restores reliable shaft sealing and fill/drain hardware function.

What should I know before ordering and installing this kit?

This kit is intended for do-it-yourself mechanics and professional technicians familiar with lower-unit tear-down and reassembly. Verify engine model, year range and mechanical-shift gearcase before purchase; some repairs may require additional bearings, seals or specialized tools not included in the kit.

• Confirm compatibility with your engine’s model/year and mechanical shift type.
• Inspect internal gearcase components during service—replace worn bearings or gears as needed.
• Use proper torque specs, sealants and grease when reinstalling components for a long-lasting repair.

Who this kit is for: SeaSierra’s 5006373 outboard gearcase seal kit suits boat owners, marina technicians, and DIYers maintaining Johnson or Evinrude outboards from 75HP to 300HP that use mechanical-shift gearcases. Choose this kit when you need a comprehensive set of common seals, O-rings, gaskets and small hardware to address leaks, carry out routine lower-unit service, or prepare an engine for extended storage or heavy use on the water.