Thermostat (120°F/50°C) – Fits Mercury Marine 6-125 HP Outboards (Replaces 14586, 18-3549)

Product Details

This replacement Thermostat (120°F/50°C) is designed for a broad range of Mercury Marine 6-125 HP 2-stroke outboards manufactured from 1986 to 2005, including standard, Sea Pro, Marathon, and Super models. It also fits various Johnson/Evinrude 4-Stroke engines with compatible mounting requirements.

It serves as a direct cross-reference for multiple OEM and aftermarket part numbers, including Mercury 14586, Sierra 18-3549, and Johnson/Evinrude 435491.

• Installs as a direct replacement for the original part.
• Constructed from durable brass with stainless steel components for corrosion resistance.
• Manufactured in an ISO9002 certified factory to ensure quality and performance.

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.

The following table details compatibility for Mercury Marine outboards. Please verify your engine’s model, horsepower, and serial number range.

This thermostat may also fit certain Johnson/Evinrude 4-stroke engines, but direct model compatibility should be confirmed by a professional technician by cross-referencing part numbers.

Technical Specs

Product Type

Outboard Engine Thermostat

Installation

Direct Replacement

Temperature Rating

120°F (48.9°C)

Operating Temperature

120°F (48.9°C) opening temperature

Material

Brass with stainless steel components

Housing Compatibility

Requires separate housing assembly (not included)

Weight

Approx. 2.08 oz

Certification

Manufactured in an ISO9002 certified factory

What’s in the Box

Your package will contain one of the following based on your selection:

When purchased as thermostat only (replaces 14586):

• 1 x Thermostat

When purchased as kit (replaces 14586A3):

• 1 x Thermostat
• 1 x Gasket (27-62386)
• 1 x Gasket (27-14318-3)
• 1 x Gasket (27-19205-1)

Installation Notes

1. Ensure the engine is completely cool and disconnect the battery for safety.
2. Locate the thermostat housing on the engine. Refer to your service manual if needed.
3. Place a drain pan underneath and carefully remove the bolts securing the housing cover.
4. Remove the old thermostat and gasket(s). Note the orientation of the thermostat for correct placement of the new one.
5. Thoroughly clean the mating surfaces of the housing and its cover, ensuring all old gasket material is removed.
6. Install the new thermostat in the same orientation as the original.
7. If your kit includes new gaskets, place them correctly on the housing.
8. Re-attach the housing cover and tighten the bolts to the torque specified by the manufacturer.
9. Reconnect the battery.
10. After installation, run the engine and carefully check for any leaks around the thermostat housing. Monitor the temperature gauge to ensure the engine warms up to the correct operating temperature.

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.