# 803665 Mercury MerCruiser Thermostat | Genuine Quicksilver OEM Part

🔧 Product Description

  

Genuine Quicksilver Thermostat - Part Number: 803665

  

Genuine Quicksilver/Mercury thermostat for selected small Mercury and Mariner outboard cooling systems. This OEM thermostat has been superseded through later Mercury service numbers and is used on specific small outboard cylinder block applications where the parts diagram specifies the 803665 / 804540 thermostat family.

  

⚙️ Key Features

  

• Genuine Quicksilver/Mercury OEM thermostat

• Used on selected small Mercury / Mariner outboard cooling systems

• Superseded to later Mercury thermostat service references

• Helps regulate engine operating temperature

• Suitable for specified 2HP-30HP small outboard applications by serial range

  

🔄 Supersession & Cross Reference

  

OEM Supersession Chain:

  

• 8M0228411 → Supersedes 803665

• 803665 earlier OEM reference

  

OEM / Related Numbers:

  

• 803665

• 8M0228411

• 804540 related thermostat reference

• 853704 related thermostat reference

• 8M0228413 related thermostat reference

• Tohatsu: 350-01030-2

• Tohatsu: 350-01030-1

• Tohatsu: 350-01030-0

  

Aftermarket / Verified Equivalents:

  

• REC: REC804540

• Aftermarket equivalents available under 804540 / 350-01030 thermostat family

  

🚤 Compatibility

  

Commonly used on:

  

• Mercury / Mariner small outboard cylinder block thermostat applications

• Mercury / Tohatsu-built small outboard cooling systems

  

Typically suits:

  

• Engine models:

• Mercury small carburetted 2HP-6HP models where 803665 / 804540 thermostat family is specified

• Mercury 15HP International 2-Cylinder, serial range 0N001590 - 0N055284

• Mercury 25HP 2-Cylinder International, serial range 0N018456 - 0N055504

• Mercury 30HP 2-Cylinder International, serial range 0N001875 - 0N055504

• Mercury / Mariner small outboard applications where the cylinder block parts diagram specifies 803665, 804540, 853704, 8M0228411 or 8M0228413

  

• HP Range: 2HP - 30HP where specified by parts diagram

• Engine Type: Mercury / Mariner small outboard cooling-system thermostat

• Year Range: Application dependent by 0N serial range and model family

  

🔌 Function

  

The thermostat controls cooling-water flow through the engine to maintain the correct operating temperature. A correct thermostat helps prevent overcooling, overheating and poor running caused by unstable engine temperature.

  

Common signs of failure:

  

• Engine overheating warning

• Engine running too cold

• Thermostat stuck open or closed

• Salt or scale build-up around thermostat

• Poor idle or slow warm-up

• Inconsistent cooling temperature

• Reduced cooling-system performance

  

📦 Condition

  

• Genuine Quicksilver part

• Brand new

• Supplied in original or equivalent packaging

## Details

- **Price:** 92.95 AUD
- **Vendor:** Quicksilver
- **Tags:** Quicksilver Marine Parts | Oils Service Kits & Engine Components

## Variants

| Variant | Price | Available |
|---------|-------|-----------|
| Default Title | 92.95 AUD | In stock |

## Images

- 803665 Mercury MerCruiser Thermostat | Genuine Quicksilver OEM Part

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> Source: [Sawley's Auto and Marine](www.sawleysautoandmarine.com.au/products/803665-mercury-mercruiser-thermostat-genuine-quicksilver-oem-part)
> Updated: 2026-05-17