Water Exhaust Shutter – Fits MerCruiser V6/V8 (Replaces 99370A2, 18-2732)
Water Exhaust Shutter – Fits MerCruiser V6/V8 (Replaces 99370A2, 18-2732) is backordered and will ship as soon as it is back in stock.
Couldn't load pickup availability
Worry-free returns
Worry-free returns
You may return most new, unopened items within 7 days of delivery for a full refund. We'll also pay the return shipping costs if the return is a result of our error (you received an incorrect or defective item, etc.).
You should expect to receive your refund within four weeks of giving your package to the return shipper, however, in many cases you will receive a refund more quickly. This time period includes the transit time for us to receive your return from the shipper (5 to 10 business days), the time it takes us to process your return once we receive it (3 to 5 business days), and the time it takes your bank to process our refund request (5 to 10 business days).
If you need to return an item, simply login to your account, view the order using the "Complete Orders" link under the My Account menu and click the Return Item(s) button. We'll notify you via e-mail of your refund once we've received and processed the returned item.
Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Water Exhaust Shutter – Fits MerCruiser V6/V8 (Replaces 99370A2, 18...
Product Details
This Water Exhaust Shutter – Fits MerCruiser V6/V8 Engines 4.3L, 5.0L, 5.7L (Replaces 99370A2) is designed for MerCruiser V6 and V8 gasoline engines from 1982–1987 equipped with 3" Y-Pipe exhaust systems. It acts as a critical check valve, preventing water from flowing back into the engine through the exhaust, which can cause significant damage.
It serves as a direct interchange for MerCruiser 99370A2, Sierra 18-2733, Mallory Marine 9-42801, GLM 89160, and Marine Pro 69370P.
- Installs directly into the Y-pipe assembly.
- Made from durable rubber with metal mounting hardware to withstand the marine environment.
- Engineered to prevent water ingestion and protect your engine.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| MerCruiser | 99370A2 |
| Sierra | 18-2733 |
| Mallory Marine | 9-42801 |
| GLM | 89160 |
| Marine Pro | 69370P |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
This shutter is designed specifically for the following MerCruiser models produced between 1982 and 1987.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| MerCruiser | V6 4.3L | 185 HP | 4-Stroke | 1983–1986 | 99370A2 | 2 BBL |
| MerCruiser | V6 4.3L | 205 HP | 4-Stroke | 1985–1986 | 99370A2 | 4 BBL |
| MerCruiser | V8 5.0L | 200 HP | 4-Stroke | 1982–1987 | 99370A2 | 2 BBL, 305 CID |
| MerCruiser | V8 5.0L | 228 HP | 4-Stroke | 1982–1987 | 99370A2 | 4 BBL, 305 CID |
| MerCruiser | V8 5.0L | 230 HP | 4-Stroke | 1982–1987 | 99370A2 | 4 BBL, 305 CID |
| MerCruiser | V8 5.0L | 898 | 4-Stroke | 1982–1987 | 99370A2 | 2 BBL, 305 CID |
| MerCruiser | V8 5.7L | 260 HP | 4-Stroke | 1982–1987 | 99370A2 | 4 BBL, 350 CID |
Technical Specs
- Product Type
- Exhaust Water Shutter/Flapper
- Installation
- Direct replacement in Y-pipe assembly
- Fuel Type
- Gasoline
- Housing Material
- Rubber with metal mounting hardware
- Dimensions
- Approximately 3" (76.2 mm) diameter
- Other Technical Attributes
- Acts as a flexible check valve to prevent water from returning up the exhaust system to the engine
What’s in the Box
- 1 x Exhaust Water Shutter (99370A2)
Installation Notes
- Ensure the engine is completely cool and the battery is disconnected.
- Gain access to the exhaust Y-pipe, which may require removing the sterndrive unit.
- Locate the old water shutter inside the Y-pipe. Note its orientation before removal.
- Remove the old shutter. It may be held by clips or pressure-fit. Clean any debris from the mounting area.
- Insert the new water shutter, ensuring it is oriented correctly to allow exhaust out while blocking water entry.
- Secure the shutter in place according to the Y-pipe design.
- Reassemble all components in reverse order of removal.
- Once reassembled, start the engine and check for any exhaust or water leaks around the service area.
Troubleshooting & FAQ
How do I know if my exhaust shutter needs replacing?
Common signs of a failing water shutter include a rattling noise from the exhaust at idle, engine overheating, or evidence of water in the exhaust manifolds or cylinders. A visual inspection may also reveal cracked, brittle, or missing shutters.
Can I install this part myself?
While installation is possible for those with mechanical experience, it often requires removal of the sterndrive to access the Y-pipe. Due to the critical function of this part, professional installation is recommended to prevent engine damage.
What happens if I don't replace a faulty water shutter?
Failure to replace a bad shutter can lead to "water reversion," where water travels back up the exhaust system and into the engine cylinders. This can cause catastrophic engine failure, including hydrolocking, corrosion, and bent connecting rods.
Is this shutter compatible with aftermarket performance exhausts?
This shutter is designed as a direct replacement for stock MerCruiser Y-pipes. Compatibility with aftermarket high-performance exhaust systems is not guaranteed. Always check the specifications of your exhaust system manufacturer.
How often should exhaust shutters be inspected or replaced?
It is good practice to inspect exhaust shutters annually or whenever the sterndrive is removed for other service. Replacement interval depends on usage (fresh vs. saltwater) and exposure, but they should be replaced immediately if they show any signs of wear, damage, or brittleness.
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
Payment methods
Your payment information is processed securely. We do not store credit card details nor have access to your credit card information.
