803665 Mercury MerCruiser Thermostat | Genuine Quicksilver OEM Part
803665 Mercury MerCruiser Thermostat | Genuine Quicksilver OEM Part
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🔧 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 This OEM thermostat has been superseded through later Mercury service numbers and is used on specific small outboard cylinder block applications where the parts diagram specifies the 803665 / 804540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- Superseded to later Mercury thermostat service references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
🚤 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
🔄 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
📦 Condition
- Genuine Quicksilver part
- Brand new
- Supplied in original or equivalent packaging
Shipping & Delivery
Shipping & Delivery
Flat-rate shipping Australia-wide — $14.99 per order, no matter how many parts you add. Orders dispatch from our Kadina, South Australia store. Need it urgently or want to collect in person? Call us on 0412 532 984 before ordering.
Returns & Warranty
Returns & Warranty
14-day change-of-mind returns — if it's not right or you ordered the wrong part, send it back within 14 days in new, unused condition for a refund or exchange. Manufacturer warranties apply to new parts. Full details in our Refunds & Returns policy.
Not sure this fits?
Not sure this fits?
Ask us before you buy — we check part numbers and fitment every day. Call 0412 532 984 (Mon–Fri 8.30am–4.30pm) or message us through the contact page with your engine or vehicle details.
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