Exhaust Flapper Protector Valve – Fits Marine Stern Drives & Inboards 5" & 6" (Replaces Sierra 18-4458)

Exhaust Flapper Protector Valve – Fits Marine Stern Drives & Inboards 5" & 6"

This external-mount exhaust flapper valve is designed for a wide range of marine stern drive and inboard engines with 5-inch and 6-inch exhaust outlets. It is compatible with major brands including MerCruiser, Chrysler, Crusader, and Pleasurecraft (PCM).

This component is a direct replacement for several common part numbers, including Sierra 18-4458, Mallory 9-44002, GLM 89260, and Volvo Penta 834438-4.

• Installation Type: Mounts externally on the exhaust outlet using the included clamp.
• Durable Material: Made with weather-resistant CR Rubber and a stainless steel housing for longevity.
• Versatile Fit: Designed for 6" outlets and adaptable to 5" outlets with two simple 45° cuts.

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.

Technical Specifications

Package Contents

• 1 x Exhaust Protector Valve (18-4458)
• 1 x Stainless Steel Clamp (6-1/16" to 7")

Installation Guide

1. Safety First: Ensure the engine is off and cool to the touch before beginning work.
2. Locate Exhaust Outlet: Access the external exhaust outlet on your boat's transom.
3. Prepare for Fit: For a 6-inch outlet, the flapper can be installed directly. For a 5-inch outlet, you must make two 45° cuts into the rubber housing as indicated on the part to adapt it for the smaller diameter.
4. Mount the Flapper: Slide the exhaust flapper valve over the exhaust port.
5. Secure the Clamp: Position the provided stainless steel clamp over the hose barb section of the flapper and tighten it securely.
6. Final Check: Ensure the flapper is firmly attached and moves freely.

Troubleshooting & FAQ

How do I know if this will fit my 5-inch or 6-inch exhaust?
This flapper is designed for both sizes. It fits a 6-inch outlet directly. For a 5-inch outlet, you must make two 45° cuts on the flapper's rubber housing before installation to ensure a proper fit.

What is the purpose of an exhaust flapper?
The flapper acts as a one-way valve, preventing water from flowing back into the engine through the exhaust system. This is especially critical when slowing down, in reverse, or in rough seas, as it protects internal engine components from water ingestion.

How can I tell if my old flapper needs replacing?
Common signs of a worn or failing flapper include visible cracks, brittleness, or missing pieces. A chattering or rattling sound from the exhaust outlet, particularly at idle, is another strong indicator that replacement is needed.

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.