Exhaust Flapper Valve & Clamp Kit – Fits 5" & 6" OD Exhaust Pipes (Replaces Sierra 18-4458, GLM 89260, Mallory 9-44002)

Exhaust Flapper Valve & Clamp Kit – Fits 5" & 6" OD Exhaust Pipes

This marine exhaust flapper valve kit is designed to prevent water from rushing in through exhaust thru-hulls when your boat is stopping, moving at slow speeds, or backing up. It is a crucial component for protecting your engine from water ingestion and potential damage. This kit is suitable for a wide range of inboard and stern drive applications with 5-inch or 6-inch outside diameter (OD) exhaust pipes.

This universal kit is a direct replacement for several common part numbers, ensuring broad compatibility with many popular engine setups. Please note that installation on a 5-inch pipe requires modification.

• Fits: 5" and 6" (127mm & 152.4mm) Outside Diameter Exhaust Pipes
• Material: Durable EPDM rubber with UV stabilizer and a marine-grade SS316 stainless steel clamp
• Package Includes: Two exhaust flapper valves and two stainless steel clamps

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

Technical Specifications

Package Contents

• 2 x Exhaust Guard Covers
• 2 x Stainless Steel Hose Clamps

Installation Guide

Follow these steps to install the exhaust flapper valve:

1. Ensure the engine is off and the exhaust system is cool to the touch.
2. For 6-inch pipes, slide the flapper valve directly over the exhaust pipe outlet.
3. For 5-inch pipes, you must first make two 45-degree cuts into the rubber housing to allow it to fit snugly.
4. Position the flapper so the valve opens away from the transom, allowing exhaust gases to exit freely.
5. Place the stainless steel clamp over the rubber housing where it meets the pipe.
6. Tighten the clamp securely with a screwdriver or socket wrench until the flapper is held firmly in place. Do not overtighten, as this could damage the rubber.
7. Visually inspect the installation to ensure a secure fit and correct orientation.

Troubleshooting & FAQ

How do I know if this flapper kit will fit my exhaust?

This kit is designed for exhaust pipes with a 5-inch or 6-inch outside diameter (OD). Measure the outer edge of your exhaust pipe to confirm the size. It is a universal fit for many inboard and I/O brands like Mercury, Chris-Craft, and Crusader using these pipe sizes.

What is the purpose of an exhaust flapper?

The flapper acts as a one-way valve on your exhaust outlet. It prevents water from surging back up the exhaust pipe when you are backing up, stopping suddenly, or in rough seas, which protects the engine from water damage.

My exhaust pipe is 5 inches. How do I modify the flapper to fit?

To fit a 5-inch OD pipe, you must make two 45-degree angle cuts into the rubber housing. This allows the rubber to slightly overlap and clamp down to the smaller pipe diameter for a secure seal.

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.