Thermostat Kit (120°) – Fits Mercury/Mariner Outboards 6-25 HP (Replaces Mercury 14586A3, Sierra 18-3549, GLM 13120, Mallory 9-43014/9-43052)

Product Details

This Mercury Mariner Thermostat Kit (120°) is designed for a wide range of Mercury and Mariner 2-stroke outboard engines from 6 to 25 HP, manufactured between 1984 and 1998. It ensures your engine maintains its optimal operating temperature for peak performance and longevity.

The kit serves as a direct replacement for multiple OEM and aftermarket parts, including Mercury 14586A3, Sierra 18-3549, GLM 13120, and Mallory 9-43014/9-43052.

• Installs directly into the engine's thermostat housing.
• Engineered to maintain a consistent 120°F operating temperature.
• Includes one thermostat and three distinct gaskets for a complete installation.

Part Number Matrix

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

Engine & Model Compatibility

Confirm your engine model/year in the table below.

For Mercury/Mariner models, check your engine’s serial number (SN) to ensure precise fitment, as different manufacturing locations (USA, Belgium, Canada) have unique serial ranges.

Technical Specs

Product Type

Thermostat Kit

Installation

Engine Mount

Fuel Type

Gasoline

Connection

Direct Replacement

Housing Material

Metal

Housing Color

Metallic Finish

Dimensions

5.91 x 3.94 x 1.18 inches (150 x 100 x 30 mm)

Weight

2.48 ounces (70.3 g)

Operating Temperature

120°F / 50°C (Note: some sources state 122°F)

Warranty

30-day replacement

What’s in the Box

• 1 x 120° Thermostat (14586)
• 1 x Gasket (27-19205002)
• 1 x Gasket (27-14318)
• 1 x Gasket (27-62386)

Installation Notes

1. Ensure the engine is completely cool and disconnect the battery for safety.
2. Locate the thermostat housing on the engine block. Place a drain pan underneath if needed.
3. Carefully remove the bolts securing the thermostat housing cover. Note their length and position if they differ.
4. Remove the old thermostat and gasket. Take note of the thermostat's orientation.
5. Thoroughly clean both the housing and cover mating surfaces. Remove all traces of old gasket material.
6. Install the new thermostat in the correct orientation, followed by the appropriate new gasket from the kit.
7. Re-secure the housing cover, tightening bolts to the manufacturer's specified torque in a star pattern.
8. Reconnect the engine's cooling hoses and refill the cooling system.
9. Reconnect the battery. Start the engine and carefully check for any leaks around the housing as it warms up.

Troubleshooting & FAQ

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

• Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
• Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
• Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.