Exhaust Flapper Valve & Clamp Kit – Fits Inboard/Stern Drive (Replaces Sierra 18-4458, GLM 89260, Mallory 9-44002)
Exhaust Flapper Valve & Clamp Kit – Fits Inboard/Stern Drive (Replaces Sierra 18-4458, GLM 89260, Mallory 9-44002) is backordered and will ship as soon as it is back in stock.
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Worry-free returns
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Delivery and Shipping
Delivery and Shipping
Orders are typically processed within 1–2 business days.
U.S. orders usually arrive within 6–15 days, depending on stock location and destination.
International delivery times may vary due to customs processing.
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Exhaust Flapper Valve & Clamp Kit – Fits Inboard/Stern Drive (R...
Exhaust Flapper Valve & Clamp Kit – Fits Inboard/Stern Drive (Replaces Sierra 18-4458, GLM 89260, Mallory 9-44002) is backordered and will ship as soon as it is back in stock.
Product Details
This Exhaust Flapper Valve & Clamp Kit – Fits 5" & 6" OD Exhaust Pipes is engineered to prevent water from entering the engine through exhaust thru-hulls during low-speed operation, stopping, or reversing. It is designed for a broad range of inboard and stern drive applications using either 5-inch or 6-inch outside diameter exhaust pipes.
It serves as a direct replacement for Sierra 18-4458, GLM 89260, and Mallory 9-44002.
- Installs externally via a clamp-on connection. Note: fitting to a 5" pipe requires two 45° cuts.
- Constructed from weather-resistant EPDM rubber with a UV stabilizer for durability in marine environments.
- Kit includes two flapper valves and two marine-grade SS316 stainless steel clamps for a complete installation.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Sierra | 18-4458 |
| GLM | 89260 |
| Mallory | 9-44002 |
| Serria/Sierra | 18-4458 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Chris-Craft | Inboard Engines | Various | 4-Stroke | For 5" & 6" exhaust pipes | ||
| Chrysler | Inboards | Various | 4-Stroke | For 5" & 6" exhaust pipes | ||
| Crusader | Inboards | Various | 4-Stroke | For 5" & 6" exhaust pipes | ||
| Mercury/Mariner | Inboard/Outboard | Various | 4-Stroke | For 5" & 6" exhaust pipes | ||
| Pleasurecraft (PCM) | Waukesha | Various | 4-Stroke | For 5" & 6" exhaust pipes |
Technical Specs
- Product Type
- Exhaust Guard / Flapper Valve & Clamp Kit
- Installation
- External; 5" requires two 45° cuts
- Connection
- Clamp-on to exhaust pipe
- Housing Material
- Weather resistant EPDM rubber with UV stabilizer
- Housing Color
- Black
- Dimensions
- Fits 5" & 6" OD pipes (127mm & 152.4mm)
- Fitting Type
- Stainless steel hose clamp
- Other Technical Attributes
- Stainless steel clamp is marine grade SS316
- Warranty
- 1 Year (YiheoZux brand) or Limited Lifetime (Sierra brand)
What’s in the Box
- 2 x Exhaust Guard Covers
- 2 x Stainless Steel Hose Clamps
Installation Notes
- Ensure the engine is completely cool and turned off before beginning installation.
- If fitting to a 5" outside diameter exhaust pipe, carefully measure and make two 45° cuts on the rubber housing to allow it to fit snugly.
- Slide the exhaust flapper valve over the end of the exhaust pipe outlet until it sits securely.
- Position the stainless steel hose clamp over the rubber housing where it meets the pipe.
- Tighten the hose clamp firmly with a screwdriver or socket wrench until the flapper is securely fastened and does not move. Do not overtighten.
Troubleshooting & FAQ
How do I know if this flapper valve will fit my boat?
This kit is designed for boats with inboard or stern drive engines that have a 5-inch or 6-inch outside diameter (OD) exhaust pipe. The best way to confirm fitment is to measure the OD of your boat's exhaust outlet pipe.
How do I install this on a 5-inch exhaust pipe?
To adapt the flapper for a 5-inch pipe, you must make two 45-degree cuts on the flexible rubber housing. This allows it to conform to the smaller pipe diameter before clamping.
What is the purpose of an exhaust flapper?
An exhaust flapper acts as a one-way valve, letting exhaust gases out while preventing water from splashing back into the exhaust system and engine when the boat is stopping, docking, or operating in reverse.
PhoneWhen should I replace my exhaust flappers?
Inspect your flappers regularly for signs of wear, such as cracking, splitting, or brittleness. It is good practice to replace them every few seasons as part of preventative maintenance, or immediately if any damage is found.
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 vs 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.
Payment & Security
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