Thermostat Kit – Fits Mercury Marine, Tohatsu/Nissan 4-Stroke 8-30 HP Outboards (Replaces 855676002, 3NV010300M)

$22.50 $36.00

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Thermostat Kit – Fits Mercury Marine, Tohatsu/Nissan 4-Stroke 8-30 HP Outboards (Replaces 855676002, 3NV010300M) is backordered and will ship as soon as it is back in stock.

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Free standard shipping on all orders under 1 KG

If you have any questions, feel free to email us at info@sierramarines.com, and we’ll get back to you within 20–36 hours.

Product Details

This Thermostat Kit – Fits Mercury Marine 4-Stroke 8-30 HP Outboards is an essential component for maintaining a stable engine operating temperature. It is engineered for Mercury Marine 8 HP to 30 HP 4-Stroke outboard engines (209cc, 2/3-cylinder Carb/EFI models) and is also compatible with select Tohatsu and Nissan 4-Stroke outboards that use the same thermostat specifications.

This kit serves as a direct interchange for Mercury part numbers 855676002, 8M0119207, 8M0139434, 855676005; Tohatsu 3NV010300M, 3R3-01030-0, 3R3010300M; and aftermarket references Sierra 18-43175 and Mallory 9-43175.

Installs as a direct replacement for the original OEM thermostat assembly.
Maintains a consistent engine operating temperature of 60°C (140°F).
Includes thermostat, cover gasket, and seals for a complete service.

Part Number Matrix

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

Brand	Part Number
Mercury	855676002
Mercury	8M0119207
Mercury	8M0139434
Mercury	855676005
Tohatsu	3NV010300M
Tohatsu	3R3-01030-0
Tohatsu	3R3010300M
Sierra	18-43175
Mallory	9-43175

Engine & Model Compatibility

Confirm your engine model/year in the table below.

This kit fits a range of Mercury Marine 4-Stroke outboards from 8 to 30 HP. Please verify your engine's part number requirements.

Also compatible with select Tohatsu and Nissan models sharing the same thermostat specifications and part numbers.

Brand	Model	Rated HP	Stroke	OEM Part No.	Notes
Mercury	4-Stroke	8	4-Stroke	855676002/8M0119207/8M0139434	209cc 2-cyl engines
Mercury	4-Stroke	9.9	4-Stroke	855676002/8M0119207/8M0139434	209cc 2-cyl engines
Mercury	4-Stroke	15	4-Stroke	855676002/8M0119207/8M0139434	209cc 2/3-cyl engines
Mercury	4-Stroke	20	4-Stroke	855676002/8M0119207/8M0139434	209cc 2/3-cyl engines
Mercury	4-Stroke	25	4-Stroke	855676002/8M0119207/8M0139434	209cc 2/3-cyl engines
Mercury	4-Stroke	30	4-Stroke	855676002/8M0119207/8M0139434	209cc 2/3-cyl engines

Technical Specs

Product Type: Thermostat Repair Kit
Installation: Direct replacement
Temperature Rating: 60°C (140°F)
Operating Temperature: 60°C (140°F)
Package Weight: 0.08 lbs (0.036 kg)

What’s in the Box

1 x Thermostat (Replaces 855676002)
1 x Thermostat Cover Gasket (Replaces 27-853702005)
2 x Thermostat Seals

Installation Notes

Ensure the engine is off and completely cool. Disconnect the negative battery cable for safety.
Place a drain pan under the engine and carefully drain the coolant from the system.
Locate the thermostat housing on the engine. It is typically a small dome-shaped cover with a hose connected to it.
Loosen and remove the bolts securing the thermostat housing cover. Note their position and length.
Carefully remove the old thermostat and its gasket or seals. Take note of the thermostat's orientation.
Thoroughly clean both the housing and cover mating surfaces, removing all traces of old gasket material and corrosion.
Install the new thermostat in the correct orientation. Place the new gasket and seals into position.
Re-install the housing cover and tighten the bolts to the manufacturer's specified torque in a crisscross pattern.
Reconnect the coolant hose and refill the cooling system with the recommended coolant type.
Reconnect the battery, start the engine, and let it warm up. Check for any leaks around the housing and monitor the engine temperature to ensure it is stable.

Troubleshooting & FAQ

How do I know if my thermostat is failing?

Common signs of a failing thermostat include engine overheating, running too cold (not reaching operating temperature), erratic temperature gauge readings, poor engine performance, and increased fuel consumption. Visual inspection may show corrosion or a stuck valve.

What happens if the thermostat is stuck open or closed?

If stuck closed, the engine will quickly overheat as coolant cannot circulate to the radiator. If stuck open, the engine will take a very long time to warm up or may never reach its optimal operating temperature, leading to poor efficiency, carbon buildup, and premature wear.

The temperature rating is listed as 140°F, but I've seen 130°F elsewhere. Which is correct?

This thermostat is rated for 60°C, which is 140°F. While some cross-references might show a slightly different rating (like 130°F), 140°F is the confirmed standard operating temperature for the specified Mercury and Tohatsu models, ensuring optimal engine performance and longevity.

How often should I replace my outboard engine's thermostat?

As a best practice for preventative maintenance, it is recommended to inspect your thermostat annually and replace it every 2-3 years or every 200-300 hours of operation, especially if you operate in saltwater environments. Always replace it if you notice signs of failure.

Can I install this thermostat kit myself?

Yes, replacement is straightforward for individuals with basic mechanical skills. However, it requires draining coolant and careful cleaning of surfaces. If you are unsure about the procedure, consulting a service manual or a qualified marine mechanic is recommended to prevent leaks or damage.

Is this a direct replacement for my OEM part?

Absolutely. This kit is designed as a direct-fit replacement. It includes the necessary thermostat, gasket, and seals to match the form, fit, and function of the original equipment manufacturer parts listed in the Part Number Matrix.

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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