Cylinder Head Gasket – Fits Tohatsu/Nissan/Mercury 25–30HP 2-Stroke (Replaces 346-01005-0)

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Cylinder Head Gasket – Fits Tohatsu/Nissan/Mercury 25–30HP 2-Stroke (Replaces 346-01005-0) is backordered and will ship as soon as it is back in stock.

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Product Details

This aftermarket Cylinder Head Gasket – Fits Tohatsu/Nissan/Mercury 25–30HP 2-Stroke (Replaces 346-01005-0) ensures a reliable seal for the engine head and cylinder interface. It is designed for Tohatsu/Nissan 25–30HP 2-Stroke engines (1996–2018) and Mercury/Mariner 25–30HP 2-Stroke engines (2002–2018), providing a direct-fit replacement for worn or damaged originals.

It is a direct interchange for Tohatsu part numbers 346-01005-0, 346-01005-1, 346-01005-2, and 346-01005-3; Mercury numbers 27-8129391, 27-8M0119224, and 8M0065584; and Sierra aftermarket part 18-2890.

Installs at the engine head and cylinder block interface.
Engineered for 2-Stroke gasoline marine engines.
Restores compression and prevents combustion gas leaks.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket brands.

Brand	Part Number
Tohatsu	346-01005-0
Tohatsu	346-01005-1
Tohatsu	346-01005-2
Tohatsu	346-01005-3
Mercury	27-8129391
Mercury	27-8M0119224
Mercury	8M0065584
Sierra	18-2890

Engine & Model Compatibility

Confirm your engine model/year in the table below.

For Tohatsu & Nissan engines, this gasket fits specific 25HP and 30HP 2-stroke models manufactured between 1996 and 2018.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Tohatsu	M25C2	25	2-Stroke	1996–2002	346-01005-0	Assembly Kit & Cylinder-Crank Case Assy
Tohatsu	M25C3	25	2-Stroke	1996–2003	346-01005-0	Assembly Kit & Cylinder-Crank Case Assembly
Tohatsu	M30A3	30	2-Stroke	1996–2002	346-01005-0	Assembly Kit & Cylinder-Crank Case Assy
Tohatsu	M30A4	30	2-Stroke	1996–2002	346-01005-0	Assembly Kit & Cylinder-Crank Case Assembly
Nissan	25HP	25	2-Stroke	1996–2018	346-01005-0	Boat engine applications
Nissan	30HP	30	2-Stroke	1996–2018	346-01005-0	Boat engine applications

For Mercury & Mariner outboards, this part is compatible with select Japanese-origin 25HP and 30HP 2-stroke models from 2002 to 2018.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercury	25HP	25	2-Stroke	2002–2018	27-8129391	Japanese Origin
Mercury	30HP	30	2-Stroke	2002–2018	27-8129391	Japanese Origin
Mariner	25HP	25	2-Stroke	2002–2018	27-8129391	Japanese Origin
Mariner	30HP	30	2-Stroke	2002–2018	27-8129391	Japanese Origin

Technical Specs

Product Type: Cylinder Head Gasket
Installation: Engine head/cylinder interface
Fuel Type: Gasoline
Engine Type: 2-Stroke
Weight: 53g (0.12 lb) or less
Dimensions: Height: 0.4 cm; Width: 15.0 cm; Length: 25.5 cm
Other Technical Attributes: Compatible with marine engines

What’s in the Box

1 x Cylinder Head Gasket (346-01005-0/346-01005-1/346-01005-2/346-01005-3)

Installation Notes

Disconnect the battery terminals and relieve fuel system pressure.
Remove the engine cowling and any components obstructing access to the cylinder head bolts, such as ignition coils or intake manifolds.
Following the service manual's sequence, loosen the cylinder head bolts in reverse order of tightening.
Carefully lift the cylinder head off the engine block. Do not pry between the mating surfaces.
Remove the old head gasket. Thoroughly clean the mating surfaces of both the cylinder head and the block to remove all old gasket material and carbon deposits.
Position the new gasket over the dowel pins on the engine block. Ensure it is oriented correctly.
Carefully lower the cylinder head back onto the block.
Install and torque the cylinder head bolts in the sequence and to the specifications outlined in your engine's service manual. This step is critical for a proper seal.
Reassemble all previously removed components.
Reconnect the fuel lines and battery, then check for any leaks before starting the engine.

Troubleshooting & FAQ

How do I know if my cylinder head gasket is failing?

Common signs of a blown head gasket include visible white smoke from the exhaust, a milky or frothy appearance in the engine oil (indicating coolant contamination), engine overheating, bubbling in the coolant reservoir, and a noticeable loss of engine power or compression.

Can I install this gasket on any Tohatsu or Mercury 30HP engine?

No. This gasket is specifically for the 2-Stroke models and year ranges listed in the compatibility tables. Always verify your full model number and year before ordering to ensure a correct fit. It is not designed for 4-Stroke or other engine series.

What is the correct way to clean the engine surfaces before installation?

Use a plastic scraper or a dedicated gasket remover solvent to get rid of all old material. Avoid using metal scrapers or abrasive pads that could gouge or scratch the aluminum surfaces of the head or block, as this can prevent a proper seal.

What is the recommended replacement interval for a head gasket?

Head gaskets do not have a fixed replacement interval and are typically only replaced when the engine is being rebuilt or if the original gasket has failed. Their lifespan depends on engine operating conditions, maintenance, and proper installation.

Is it necessary to use a sealant with this gasket?

Most modern head gaskets, including this one, are designed to be installed dry without any additional sealants. Applying sealant can interfere with the gasket's ability to crush and seal correctly. Always follow the specific instructions in your engine's service manual.

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

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