Thermostat (60°C/140°F) – Fits Honda, Mercury & Yamaha 8-255 HP (Replaces 19300-ZW9-003, 6G8-12411-00)

Product Details

This Thermostat (60°C/140°F) – Fits Honda, Mercury & Yamaha 8-255 HP is a direct-fit aftermarket replacement for a wide range of outboard motors. It is engineered for Honda 4-stroke engines from 8–225 HP (2001-Present), Mercury 4-stroke engines from 15–90 HP, and Yamaha 4-stroke engines from 9.9–70 HP (1987-Present).

The part serves as a direct interchange for Honda 19300-ZW9-003/19300-ZY3-023, Yamaha 6G8-12411 and 66M-12411 series, Mercury 825212/855676 series, and Sierra 18-3541.

• Engineered for direct OEM-replacement installation.
• Constructed from OEM-grade brass and rubber composite for durability.
• Opens at a factory-rated 60°C (140°F) to ensure proper engine operating temperature.

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 Honda Owners: Fits a broad range of 4-stroke models from 2001 to present. Confirm your specific HP rating in the table.

For Mercury Owners: Applications cover select 15, 75, and 90 HP 4-stroke models. Verify your OEM part number, as specific model years were not provided in source data.

For Yamaha Owners: Compatible with various 4-stroke models from 1987 onward. Cross-reference your original part number with the matrix above to ensure correct fitment.

Technical Specs

Product Type

Marine Engine Thermostat

Installation

Direct OEM Replacement

Housing Material

OEM-grade brass and rubber composite

Operating Temperature

60°C (140°F) Opening Temperature

Construction

Aftermarket replacement with OEM-equivalent construction

What’s in the Box

• 1 x Thermostat Assembly (60°C/140°F rating)

Installation Notes

1. Ensure the engine is completely cool before starting work.
2. Disconnect the negative battery terminal as a safety precaution.
3. Locate the thermostat housing, typically on the engine block or cylinder head.
4. Place a drain pan beneath the housing to catch any spilling coolant.
5. Carefully unbolt and remove the thermostat housing cover.
6. Take out the old thermostat, noting its orientation.
7. Thoroughly clean all old gasket material from the housing and cover mating surfaces.
8. Install the new thermostat in the same orientation as the original.
9. Place a new gasket on the housing before reattaching the cover.
10. Tighten the housing bolts to the manufacturer's recommended torque specifications.
11. Reconnect the battery, refill the cooling system, and check for leaks after starting the engine.

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.