Yamaha Outboard Lower Unit Drain Gasket – Fits Yamaha 4-Stroke Outboards (Replaces 90430-08003-00)

Yamaha Outboard Lower Unit Drain Gasket – Fits Yamaha 4-Stroke Outboards (Replaces 90430-08003-00)

This lower unit drain gasket fits a wide range of Yamaha 4-Stroke Outboard Motors, from 2.5 HP to 350 HP, model years 1998 to present. It is designed for use on both the lower unit drain plug and the upper oil level vent screw, creating a secure seal to prevent gear lube leaks.

This part is a direct replacement for Yamaha 90430-08003-00 and 90430-08020-00, as well as Sierra 18-0826 and Mercury 27-81987.

• Application: Used for both the drain plug and vent screw in the lower unit.
• Material: High-quality fiber/composite designed for marine environments.
• Installation: Requires only basic hand tools for a secure fit.

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 Yamaha owners, here’s a guide to help you identify your engine’s serial/model: Yamaha Serial Number Guide. You can also review exploded diagrams to verify part numbers: Yamaha Parts Diagrams & Catalog.

Technical Specifications

Package Contents

This package includes:

• 1 x Gasket (90430-08003-00)

Note: This gasket is used on both the vent and drain plug. For a complete lower unit oil change, please order a quantity of two.

Installation Guide

1. Position a suitable drain pan under the lower unit of your outboard.
2. Using a large flathead screwdriver or a socket wrench, carefully loosen and remove the lower drain plug.
3. Loosen and remove the upper vent screw to allow air to enter and the oil to drain smoothly.
4. Allow the gear lube to drain completely. Inspect the old oil for any metal shavings or a milky appearance, which could indicate water intrusion.
5. Remove the old gaskets from both the drain plug and the vent screw. Clean the threads and sealing surfaces on the lower unit to ensure a proper seal.
6. Fill the lower unit with the manufacturer-recommended gear lube through the bottom drain hole. Continue filling until lube begins to flow out of the upper vent hole.
7. Install a new gasket on the upper vent screw and tighten it securely. Do not overtighten.
8. Quickly install a new gasket on the lower drain plug and tighten it to the manufacturer's specified torque. Using a torque wrench is highly recommended to prevent leaks.
9. Wipe the area clean and check for any signs of leaks around both plugs.

Troubleshooting & FAQ

The new black gasket feels harder than my old red one. Is this correct?
Yes, this is normal. The current OEM part (90430-08003-00) is a black, harder fiber/composite material that supersedes the previous reddish-orange version (90430-08020-00). Our aftermarket gasket is manufactured to match the current specification.

My gasket seems to be leaking or loosening after installation. What should I do?
Leaks or loosening are often caused by incorrect torque. These gaskets are crush washers and must be tightened to the manufacturer's specification. Over-tightening can damage the gasket or plug, while under-tightening will fail to create a proper seal. We recommend using a torque wrench. If leaks persist, inspect the sealing surfaces on your lower unit and plugs for nicks or damage.

How many gaskets do I need for a lower unit oil change?
You will need two gaskets for a complete service—one for the lower drain plug and a second for the upper vent screw. This listing is for a single gasket.

Can I reuse the old gasket?
No. It is strongly recommended to use a new gasket every time you change your lower unit oil. Reusing a compressed crush washer significantly increases the risk of an improper seal, which can lead to catastrophic gear lube leaks and lower unit failure.

OEM vs. Aftermarket?
This listing is an aftermarket replacement part (not an OEM component). We build to OEM specifications using comparable-grade materials. Production is done in an ISO9002-certified facility, and each batch is inspected for consistent performance.

How fast do you ship and how much does it cost?
We offer fast global shipping—typically 5–15 days worldwide—and standard shipping is free. Expedited options are available at checkout depending on your location.

What is your warranty and return policy?
This product includes a 1-year warranty against manufacturing defects under normal use. If something goes wrong, contact our support team with your order details. Returns are accepted within our standard return window (unused & in original condition); please see our policy page or reach out for instructions.

Important Notes

• Verify that the product images and specifications match your original part. If discrepancies exist—even if your model is listed—do not proceed with the purchase.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.

Why Choose JLM MARINE?

• Quality Assured: As a trusted manufacturer for over 20 years, we supply top marine brands globally.
• Fast Global Shipping: Free delivery worldwide in 5–15 days.
• Direct from Manufacturer: Competitive pricing without compromising quality.