Lower Unit Fill Drain Plug Washer – Fits Johnson Evinrude OMC 20-300HP (Replaces 311598, 18-4248, 9-60408)

Product Details

This Lower Unit Fill Drain Plug Washer is engineered for Johnson Evinrude OMC outboard engines ranging from 20 to 300 HP. Compatible with a wide range of 2-cylinder, 3-cylinder, V4, and V6 configurations spanning model years 1975 through 2006, this drain screw gasket ensures a reliable seal during lower unit service intervals.

Direct aftermarket replacement for OEM part numbers 311598, 18-4248, 9-60408, and 3852105 across Johnson/Evinrude Outboard, OMC Stern Drive, Mallory, and Volvo Penta applications.

• Seals lower unit fill and drain plug points to prevent gear oil leakage
• Designed for both 2-stroke and 4-stroke outboard applications
• Replace during every lower unit oil change for optimal protection

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket.

Engine & Model Compatibility

Confirm your engine model/year in the table below.

Technical Specs

OEM Part Number

311598

Interchange Numbers

18-4248, 9-60408, 3852105

Application

Lower Unit Fill & Drain Plug Seal

Fitment Range

Johnson Evinrude OMC 20–300 HP Outboards

Model Years

1975–2006

What's in the Box

• 1× Gasket 311598

Installation Notes

1. Ensure engine is off and lower unit is cool to the touch before beginning service.
2. Position boat or tilt engine so lower unit is accessible and level.
3. Remove the existing drain plug and allow gear oil to fully drain into a suitable container.
4. Inspect the old washer for signs of wear, compression set, or cracking.
5. Clean the plug threads and sealing surface on the lower unit housing.
6. Place the new washer onto the drain plug, ensuring it seats evenly.
7. Reinstall the drain plug and tighten to manufacturer-specified torque to prevent over-compression or leakage.
8. Refill the lower unit with the correct grade and quantity of gear oil per your service manual.
9. Install the fill plug with a new washer using the same procedure.
10. Run the engine briefly, then inspect both plugs for any signs of seepage.

Troubleshooting & FAQ

How often should I replace this washer?

Replace the washer every time you perform a lower unit oil change—typically every 100 hours of operation or annually, whichever comes first. Reusing old washers increases the risk of leaks and contamination.

Will this fit both the fill and drain plugs?

Yes, the same washer design is used for both the fill and drain plug positions on compatible Johnson Evinrude lower units. Always replace both washers during each service interval.

What are the signs of a failing drain plug washer?

Visible gear oil seepage around the plug, low oil levels during inspection, or milky oil indicating water intrusion are common indicators. If you notice any of these, replace the washer immediately and inspect for corrosion or thread damage.

Can I reuse the washer if it looks okay?

No. Even if the washer appears intact, it has been compressed and may not seal reliably after removal. Always use a new washer to ensure a proper seal and prevent costly lower unit damage.

What torque spec should I use when reinstalling the plug?

Consult your engine's service manual for exact torque values, as they vary by model. Over-tightening can crush the washer and damage threads; under-tightening may cause leaks.

Is this compatible with OMC Cobra stern drives?

Yes, this washer crosses to OMC Stern Drive part numbers 311598 and 3852105. Verify your stern drive model and year against the compatibility table to confirm fitment.

OEM vs. Aftermarket?

This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.

Shipping speed & cost

We ship worldwide quickly—typical transit 5–15 days. Standard shipping is free; expedited options appear at checkout where available.

Warranty & returns

Covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window (unused & in original condition). See our policy page or reach out for instructions.

Safety & Important Info

• Verify images/specifications against your original part. If anything differs—even if your model appears compatible—do not purchase.
• This is an aftermarket replacement, not an OEM component.
• Professional installation is recommended for safety and performance.

About Sea Sierra

• Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
• Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
• Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.