Thermostat Kit, 140° – Fits MerCruiser Applications (Replaces 807252Q5 and 807252T8)
Thermostat Kit, 140° – Fits MerCruiser Applications (Replaces 807252Q5 and 807252T8) is backordered and will ship as soon as it is back in stock.
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Thermostat Kit, 140° – Fits MerCruiser Applications (Replaces 80725...
Thermostat Kit, 140° – Fits MerCruiser Applications (Replaces 807252Q5 and 807252T8) is backordered and will ship as soon as it is back in stock.
Product Details
This 140° thermostat kit is a replacement cooling-system service kit for selected MerCruiser inboard and sterndrive applications. It is associated with Mercury Marine reference numbers 807252Q5 and 807252T8, and is cross-referenced in the Sierra catalog as thermostat kit 18-3647.
The kit helps restore the thermostat and gasket arrangement used by the original cooling system. Because MerCruiser fitment can vary by engine configuration and serial-number range, compare the existing thermostat, gasket layout, and engine identification details before ordering.
- Aftermarket thermostat kit for selected MerCruiser cooling systems.
- 140°F temperature rating as listed for the related thermostat application.
- Kit format includes the thermostat and multiple housing or cover gaskets for service installation.
- Use the reference numbers and component list below to confirm compatibility.
Part Number Matrix
These are reference numbers associated with this thermostat kit. They should be used for identification and cross-reference only; always confirm the complete engine application before installation.
| Brand or Catalog | Reference Number | Reference Type |
|---|---|---|
| Mercury Marine | 807252Q5 | Replacement reference |
| Mercury Marine | 807252T8 | Replacement reference |
| Sierra | 18-3647 | Thermostat kit reference |
Engine and Model Compatibility
This kit is intended for selected MerCruiser cooling-system applications associated with the references above. The available catalog information does not establish one universal year, horsepower, or serial-number range for every MerCruiser V6 or V8 engine. Confirm the engine model, cooling-system configuration, and serial-number range against the service manual or the original part before purchase.
| Application | Cooling-System Use | Temperature | Reference Numbers | Fitment Note |
|---|---|---|---|---|
| Selected MerCruiser inboard and sterndrive applications | Verify against the original thermostat and gasket arrangement | 140°F | 807252Q5, 807252T8, 18-3647 | Model, year, configuration, and serial range must be confirmed before installation |
Technical Specifications
- Product Type
- Thermostat kit with housing and cover gaskets
- Temperature Rating
- 140°F
- Application
- Selected MerCruiser cooling-system applications
- Installation
- Replacement service installation using the original housing and fasteners
- Kit Reference
- Sierra 18-3647
- Replacement References
- Mercury Marine 807252Q5 and 807252T8
- Fitment Limitation
- Confirm model, cooling-system configuration, and serial-number range before ordering
What's in the Box
Catalog-listed components for the 18-3647 kit include:
- 1 x Thermostat, reference 18-3551
- 1 x Thermostat housing gasket, reference 18-2917
- 1 x Thermostat cover gasket, reference 18-2844
- 1 x Thermostat cover gasket, reference 18-2553
- 1 x Thermostat cover gasket, reference 18-2834
Gasket shapes and openings must match the housing being serviced. Do not combine gaskets from a different thermostat kit or application.
Installation Notes
- Allow the engine to cool completely before beginning work. Follow the manufacturer's safety procedures.
- Disconnect the battery if required by the service procedure and close or isolate any water or coolant supply as appropriate.
- Locate the thermostat housing and drain enough coolant or water to prevent spillage during removal.
- Remove the housing or cover fasteners and carefully separate the mating surfaces.
- Remove the old thermostat and gasket material. Record the thermostat orientation before removal.
- Clean the mating surfaces without damaging the housing or engine sealing faces.
- Install the new thermostat in the same orientation as the original and select the gasket that matches the housing configuration.
- Reassemble the housing and tighten the fasteners to the engine manufacturer's specified torque.
- Restore the cooling system, start the engine, and check operating temperature and the housing area for leaks.
Troubleshooting and FAQ
What temperature thermostat does this kit use?
The related thermostat application is listed at 140°F. Confirm that this temperature rating matches your engine's required specification before installation.
What part numbers does this kit replace?
The catalog reference for this kit lists Mercury Marine numbers 807252Q5 and 807252T8. Sierra catalog reference 18-3647 identifies the thermostat kit.
Will this fit every MerCruiser V6 or V8 engine?
No universal V6 or V8 fitment should be assumed. MerCruiser applications vary by model, cooling-system configuration, and serial-number range. Compare your original components and verify the service-manual application before ordering.
Why are several gaskets included?
The kit covers the gasket arrangement associated with the listed application. Use only the gasket or gaskets that match your thermostat housing and cover. Do not install every gasket automatically if the original configuration uses a different arrangement.
What are common symptoms of a thermostat problem?
Possible symptoms include overheating, unstable operating temperature, slow warm-up, or coolant or water leakage near the thermostat housing. These symptoms can also be caused by an impeller, hose, heat exchanger, exhaust-manifold, or cooling-passage problem, so inspect the complete cooling system.
Is this an OEM component?
This listing is for an aftermarket replacement kit. Mercury Marine and Sierra numbers are provided as reference numbers for identification and cross-reference, not as a statement that the replacement is manufactured by Mercury Marine or Sierra.
Safety and Important Information
- Verify the engine model, serial-number range, cooling-system configuration, and original gasket layout before ordering.
- Do not rely on horsepower, displacement, or model-family names alone to confirm fitment.
- Install only after the engine has cooled and follow the applicable marine service procedure.
- Use the correct gasket for the housing being serviced and inspect all sealing surfaces before reassembly.
- Professional installation is recommended when the application or cooling-system configuration is uncertain.
About Sea Sierra
- Marine Parts Selection — We supply replacement components for a range of marine cooling and engine-service applications.
- Application-Focused Information — Reference numbers and component details are provided to help customers compare replacement parts accurately.
- Support for Installation Planning — Always confirm model and serial-number details before ordering or beginning service.
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