Thermostat Housing Gasket – Fits Mercury Mercruiser Ford V8 Engines (Replaces 27-54506)

$1.49

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Product Details

This Thermostat Housing Gasket – Fits Mercury Mercruiser Ford V8 Engines (Replaces 27-54506) is designed for a precise fit on Mercury Mercruiser Ford V8 302/351 engines from 1970–1977. It creates a reliable seal specifically for raw water cooled applications, preventing coolant leaks and ensuring proper engine operating temperature.

This gasket serves as a direct replacement for Mercury Marine 27-54506, Sierra 18-0349, and PCM RM0001, offering a reliable interchange for multiple part numbers.

Installs between the lower thermostat housing and the engine intake manifold.
Engineered specifically for raw water (seawater) cooling systems.
Includes one gasket for a complete replacement.

Part Number Matrix

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

Brand	Part Number
Mercury Marine	27-54506
Sierra	18-0349
PCM	RM0001

Engine & Model Compatibility

Confirm your engine model/year in the table below.

For Mercruiser models, verify your engine's serial number falls within the specified range for a guaranteed match.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercruiser	215 (4 BBL.) FORD 302 V-8	215	4-Stroke	1970–1972	27-54506	Serial #2781153 through #3395720
Mercruiser	215 MIE FORD 302 V-8	215	4-Stroke	1970–1972	27-54506	Serial #2781153 through #3563181
Mercruiser	225 (4 BBL.) FORD 302 V-8	225	4-Stroke	1973–1974	27-54506	Serial #3385721 through #4166999
Mercruiser	225 MIE FORD 302 V-8	225	4-Stroke	1973–1974	27-54506	Serial #3385721 through #4176999
Mercruiser	225-S (4 BBL.) FORD 302 V-8	225	4-Stroke	1973–1974	27-54506	Serial #3385721 through #4166999
Mercruiser	233 FORD 351 V-8	233	4-Stroke	1975–1977	27-54506	Serial #4173000 through #4902304
Mercruiser	233 MIE FORD 351 V-8	233	4-Stroke	1975–1976	27-54506	Serial #4173000 through #4775784
Mercruiser	255 FORD 351 V-8	255	4-Stroke	1973–1974	27-54506	Serial #3258728 through #4175499
Mercruiser	255 MIE FORD 351 V-8	255	4-Stroke	1973–1974	27-54506	Serial #3258728 through #4178299
Mercruiser	888 (2 BBL.) FORD 302 V-8	188	4-Stroke	1971–1977	27-54506	Serial #3108723 and up

Technical Specs

Product Type: Thermostat Housing Gasket
Installation: Fits between lower thermostat housing and engine intake manifold
Fuel Type: Gasoline
Housing Material: Gasket Material
Dimensions: 11.5 × 11.0 × 0.1 inches
Weight: 0.01 lbs (4.5 g)
Other Technical Attributes: For raw water cooled applications only

What’s in the Box

1 × Thermostat Housing Gasket (27-54506)

Installation Notes

Disconnect the vessel's battery and ensure the engine is completely cool.
Drain the engine block's cooling system to a level below the thermostat housing.
Carefully remove the bolts securing the thermostat housing, and detach any connected hoses.
Remove the old housing and gasket. Thoroughly clean both mating surfaces on the housing and the intake manifold, ensuring they are free of old gasket material, sealant, and corrosion.
Position the new gasket onto the manifold, aligning it with the bolt holes.
Place the thermostat housing over the gasket, re-insert the bolts, and tighten them to the manufacturer's specified torque in a star or crisscross pattern.
Reconnect hoses, refill the cooling system with the appropriate coolant, and check for leaks.
Reconnect the battery, then run the engine and monitor for any leaks or overheating issues.

Troubleshooting & FAQ

How do I know if my thermostat housing gasket is failing?

Signs of a failing gasket include visible coolant leaks around the thermostat housing, engine overheating, or a low coolant level warning. You may also notice white steam or a sweet smell from the engine compartment.

Can I reuse my old gasket after removing the housing?

No. Gaskets are designed for one-time use. They compress and deform to create a seal, and once removed, they will not seal properly again, leading to leaks. Always replace the gasket whenever the thermostat housing is removed.

Is any sealant required for this gasket?

Most modern gaskets of this type are designed to be installed dry. However, always consult your engine's service manual. If sealant is recommended, use only the specified type and apply it sparingly to avoid blocking coolant passages.

What does "for raw water cooled applications only" mean?

This means the gasket material is formulated to withstand direct exposure to seawater or lake water used for engine cooling. It is not intended for "closed cooling" systems that use a treated antifreeze mixture.

What is the difference between this aftermarket gasket and an OEM one?

This aftermarket gasket is engineered to meet or exceed OEM specifications for fit, form, and function. While the exact material composition may differ slightly, it is designed to provide a comparable seal and service life under the same operating conditions.

How often should this gasket be replaced?

The gasket should be replaced any time the thermostat is serviced or the housing is removed. There is no fixed replacement interval, but it's good practice to inspect it for leaks during routine engine maintenance.

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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