Thermostat Gasket – Fits MerCruiser V8 Engines (Replaces 27-418122)

$13.99 $22.00

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Product Details

This Thermostat Gasket – Fits MerCruiser V8 Engines (Replaces 27-418122) is engineered for various MerCruiser high-performance V8 engines, including the 420/425, 450, 465, 500, 525, and Scorpion 377 models from 1987 through the early 2000s. It ensures a reliable seal for your engine's cooling system.

It serves as a direct replacement for Mercury/MerCruiser part numbers 27-418122 and 418122.

Designed for direct replacement, ensuring a perfect fit and seal.
Constructed from high-quality, corrosion-resistant materials for durability in marine environments.
Engineered to maintain a tight seal for the thermostat housing, preventing coolant leaks.

Part Number Matrix

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

Brand	Part Number
Mercury/MerCruiser	27-418122
Mercury/MerCruiser	418122

Engine & Model Compatibility

Confirm your engine model/year in the table below.

This gasket is designed for a wide range of high-performance MerCruiser and Mercury Racing V8 engines. Please check your specific model and serial number range to ensure correct fitment.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercury Racing	420 GM 454 V-8	420	4-Stroke	1987–1989	27-418122	Serial 0A398941 THRU 0B622418
Mercury Racing	425 GM 454 V-8	425	4-Stroke	1990–1992	27-418122	Serial 0A398941 THRU 0D937011
Mercury Racing	425 (GEN. V) GM 454 V-8	425	4-Stroke	1992–1993	27-418122	Serial 0A398941 THRU 0D937011
Mercury Racing	450 (GEN. V) GM 454 V-8	450	4-Stroke	1995–1996	27-418122	Serial 0F347100 THRU 0F347277
Mercury Racing	465 GM 502 V-8	465	4-Stroke	1990–1992	27-418122	Serial 0C849992 THRU 0F213244
Mercury Racing	465 (GEN. V) GM 502 V-8	465	4-Stroke	1992–1994	27-418122	Serial 0C849992 THRU 0F213244
Mercury Racing	500 (GEN. +) GM 502 V-8	500	4-Stroke	1999	27-418122	Serial 0L414639 THRU 0L601036
Mercury Racing	500 (GEN. V) GM 502 V-8	500	4-Stroke	1994–1995	27-418122	Serial 0F213293 THRU 0F615766
Mercury Racing	500 (GEN. V) GM 502 V-8	500	4-Stroke	1996	27-418122	Serial 0F615767 THRU 0F745159
Mercury Racing	500 (GEN. VI) GM 502 V-8	500	4-Stroke	1997–1998	27-418122	Serial 0F745195 THRU 0F877722, 0K000001 THRU 0L000737, 0L000768 THRU 0L001772
Mercury Racing	500 Bulldog GM 540 V-8	500	4-Stroke	1990–1996	27-418122	Serial 0C577254 THRU 0F745405
Mercury Racing	500 Bulldog	500	4-Stroke	1999–2001	27-418122	Serial 0L414620 THRU 0M050281
Mercury Racing	500 EFI	500	4-Stroke	Various	27-418122	Multiple Serial Ranges
Mercury Racing	525 SC (GEN. V) GM 454 V-8	525	4-Stroke	1992–1995	27-418122	Serial 0D456000 THRU 0F348963
Mercury Racing	525 SC (GEN. V) GM 454 V-8	525	4-Stroke	1996	27-418122	Serial 0F615814 THRU 0F745215
Mercury Racing	525 SC GM 454 V-8	525	4-Stroke	1991–1992	27-418122	Serial 0D456000 THRU 0F348963
Mercury Racing	525 SC Gen VI	525	4-Stroke	Various	27-418122	Serial 0F745260 THRU 0F877734, 0K000012 THRU 0L415623
Mercury Racing	Scorpion 377 Sterndrive	377	4-Stroke	Various	27-418122	Serial 0L416146 THRU 0M907201
Mercury Racing	540 Stroker 4V	540	4-Stroke	Various	27-418122	Serial 1G400000 & Up

Technical Specs

Product Type: Thermostat Gasket
Installation: Direct Replacement
Fuel Type: Gasoline
Housing Material: Corrosion-Resistant Material (Marine Grade)
Dimensions: 15.5" L x 10.0" W x 0.3" H
Operating Temperature: Designed for Marine Engine Cooling Systems
Other Technical Attributes: Ensures tight seal for thermostat housing

What’s in the Box

1 x Gasket (27-418122)

Installation Notes

Ensure the engine is completely cool and disconnect the battery terminals for safety.
Drain the engine coolant to a level below the thermostat housing.
Carefully remove the bolts that secure the thermostat housing cover.
Take out the old thermostat and gasket. Thoroughly clean both mating surfaces of any old gasket material or residue.
Position the new thermostat and gasket correctly.
Re-install the thermostat housing cover and tighten the bolts to the manufacturer's specified torque.
Refill the cooling system with the appropriate coolant, reconnect the battery, and start the engine to check for any leaks.

Troubleshooting & FAQ

How do I know if this gasket will fit my engine?

This gasket fits a specific range of MerCruiser engines. Please refer to the "Engine & Model Compatibility" chart and cross-reference your engine's model, year, and serial number. Verifying the OEM part number (27-418122) is the most reliable method.

Do I need to use a sealant with this gasket?

Generally, this type of gasket is designed to be installed dry. However, always consult your engine's service manual for specific recommendations. Some mechanics apply a very thin layer of marine-grade gasket sealant as a precaution.

What are the signs of a failing thermostat gasket?

Common symptoms include visible coolant leaks around the thermostat housing, engine overheating, or an engine that takes too long to reach operating temperature. You may also notice a sweet smell of antifreeze after the engine has been running.

How often should I replace the thermostat gasket?

It is best practice to replace the thermostat gasket whenever you replace the thermostat, or if you have to remove the housing for any other service. There is no fixed replacement interval, but it should be inspected regularly for signs of wear or leakage.

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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Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.

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