Mercury MerCruiser - Heat Exchanger Kit 3.0L Side Mount MC-342 - 311342
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The MC-342 Fresh Water Closed Cooling Kit, part number 311342, is a half-system closed cooling conversion kit for MerCruiser 3.0L and 3.0LX GM 181 CID inline-4 marine engines. This kit converts the engine block from raw-water cooling to a closed-loop antifreeze/coolant system, helping protect the block from internal salt, rust, scale, and corrosion buildup.
Key Features
- Fresh water closed cooling conversion kit
- Model: MC-342
- Part number: 311342
- Half-system design for engine block cooling
- Side-mount heat exchanger layout
- Designed for MerCruiser 3.0L / 3.0LX GM 181 CID inline-4 engines
- Copper-alloy heat exchanger construction
- Includes mounting hardware, hoses, clamps, thermostat housing spacer, and coolant recovery bottle
Engine Compatibility
Fits MerCruiser 3.0L and 3.0LX GM 181 CID inline-4 marine engines, commonly used in 1986–2002 applications.
Serial Number Fitment Notice
Verify fitment by engine serial number, cooling-system layout, manifold/riser configuration, and available engine-bay clearance before ordering. This is a half-system kit, so the engine block is converted to closed cooling while raw water continues to cool the exhaust manifold and riser.
When This Part Needs Replacement
Install this kit when converting a raw-water cooled MerCruiser 3.0L engine to closed cooling, especially for saltwater use or engines where long-term internal block corrosion protection is a priority.
Installation Notes
This side-mount kit is designed for tighter engine bays where a top-mount exchanger may not clear the hatch or deck. Installation typically includes mounting the exchanger to the side of the engine, routing molded coolant hoses, installing the thermostat housing spacer, adding the recovery bottle, and filling/bleeding the closed cooling loop with the correct coolant mixture.
Alternate/Superseded Part Numbers
Common references: MC-342 and 311342.
Parts Desk Insight
A half-system kit protects the engine block, but it does not convert the exhaust manifold and riser to closed cooling. Those components still see raw water, so they should remain on a normal inspection and replacement schedule.
Shipping: Locally warehoused, may take up to 3 days to ship. ⚓
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 3–4 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
Returns & Satisfaction Guarantee
We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
We are serious about product integrity — and equally serious about taking care of our customers. If there is ever a product issue, contact us immediately so we can make it right.
For return authorization, please contact our support team before shipping anything back.