120°F (48.9°C) Outboard Thermostat for Mercury Marine 6–125 HP 2-Stroke Outboards – Replaces 14586 and 18-3549

Product Details

This replacement 120°F (48.9°C) outboard thermostat is intended for a broad range of Mercury Marine 6–125 HP 2-stroke outboards manufactured from 1986 to 2005, including standard, Sea Pro, Marathon, Super, and selected Jet models listed below. Always confirm the complete engine model, year, and serial-number range before ordering.

This aftermarket thermostat is listed as a cross-reference for several OEM and aftermarket numbers, including Mercury 14586 and Sierra 18-3549. Cross-reference numbers are provided for identification; they do not replace verification of the engine model and serial-number conditions.

• Direct replacement design for the original thermostat where the listed application and mounting requirements match.
• Brass construction with stainless-steel components for marine operating conditions.
• Manufactured in an ISO9002-certified factory according to the supplied product information.
• This listing is for one thermostat; the housing and gaskets are not included.

Part Number Matrix

Use the matrix below as a cross-reference aid. Confirm the original part and engine application before installation.

Engine & Model Compatibility

The compatibility information below applies only when the model, horsepower, stroke, year, and serial-number conditions all match. Verify your engine identification plate and service-manual information before ordering.

Technical Specs

Product Type

Outboard engine thermostat

Installation

Direct replacement where application conditions match

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 the original or compatible thermostat housing assembly; housing not included

Weight

Approximately 2.08 oz

Certification

Manufactured in an ISO9002-certified factory according to the supplied product information

What’s in the Box

• 1 × 120°F (48.9°C) thermostat

Not included: thermostat housing, housing cover, or replacement gaskets.

Installation Notes

1. Make sure the engine is completely cool and disconnect the battery for safety.
2. Locate the thermostat housing and consult the engine service manual for access and model-specific procedures.
3. Place a drain pan underneath and carefully remove the bolts securing the housing cover.
4. Remove the old thermostat and gasket. Note the thermostat orientation before removal.
5. Clean the housing and cover mating surfaces, removing all old gasket material without damaging the sealing surfaces.
6. Install the new thermostat in the same orientation as the original.
7. Install a new, separately sourced gasket suitable for the specific engine and housing.
8. Reattach the housing cover and tighten the bolts to the manufacturer’s specified torque.
9. Reconnect the battery.
10. Run the engine according to the service manual and check carefully for water leaks around the thermostat housing. Confirm that the engine reaches and maintains the expected operating temperature.

Troubleshooting & FAQ

Safety & Important Information

• Verify the engine model, year, horsepower, stroke, serial number, original thermostat, and housing before ordering.
• Do not rely on horsepower alone to determine fitment.
• Compare the replacement part with the original. Do not install it if the dimensions, configuration, or application do not match.
• This is an aftermarket replacement, not an OEM component.
• Allow the engine to cool completely before servicing the thermostat housing.
• Professional installation is recommended for safe and correct operation.

About Sea Sierra

• Pro-Grade Standards — We supply marine parts selected for dependable performance in real-world conditions.
• Global Fulfillment — Shipping options and delivery estimates are provided during checkout.
• Manufacturer-Direct Value — Fair pricing with attention to materials and quality control.