Sierra 18-3647 MerCruiser 160°F Thermostat & Gasket Kit
18-3647 MA104
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Sierra 18-3647 MerCruiser 160°F Thermostat & Gasket Kit
Aftermarket service kit for specified 1987-up GM-based MerCruiser cooling systems
Sierra 18-3647 is a comprehensive aftermarket thermostat and gasket kit centred on the 18-3649 thermostat rated at 160°F / 71.1°C. It is catalogued for compatible 1987-and-newer GM-based MerCruiser engines whose cooling-system parts list calls for the corresponding Mercury kit/reference.
Key Features
- 160°F / 71.1°C thermostat
- Includes multiple housing/cover gaskets and O-ring for supported configurations
- Aftermarket Sierra marine service kit
- For compatible 1987-up GM-based MerCruiser applications
- Replaces specified Mercury thermostat-kit references
Technical Specs
| Sierra kit | 18-3647 |
| Thermostat | 18-3649 |
| Opening temperature | 160°F / 71.1°C |
| Kit components | 18-2552, 18-2553, 18-2844, 18-2917, 18-0398 gaskets; 18-7173 O-ring; 18-3649 thermostat |
| 4Boats SKU | 18-3647 MA104 |
Compatibility / Fitment Guide
Confirmed catalogue basis: 1987-and-newer GM-based MerCruiser inboard/sterndrive engines with the standard thermostat housing/cooling arrangement that specifically requires a 160°F thermostat and cross-references Mercury 807252Q5 / 807252T5. Depending on the exact housing, only the applicable gaskets from this broad kit are used.
Verification required: identify engine serial number, cooling type (raw-water or closed/freshwater), thermostat temperature stamped on the old part, housing shape and OEM parts-diagram number. Displacements including 3.0L, 4.3L and GM V8 families span multiple cooling configurations, so displacement/year alone is insufficient.
Do not use when: the engine calls for a 140°F thermostat, a different housing/gasket set, a non-GM MerCruiser package or an unmatched Mercury reference.
Part Number Cross Reference
| Sierra kit | 18-3647 / display pack 18-3647D |
| Thermostat element | 18-3649 |
| Mercury / MerCruiser kit | 807252Q5 / 807252T5 |
| Mercury thermostat reference | 807252-2 |
| Mallory | 9-43157 |
| 4Boats SKU | 18-3647 MA104 |
Frequently Asked Questions
Will it fit every MerCruiser 4.3L or 5.7L?
No. Check serial number, cooling type, housing and the OEM kit number; these engines used different thermostat systems.
What temperature is the thermostat?
160°F / 71.1°C.
Why are there several gaskets?
18-3647 covers multiple supported GM-based MerCruiser housing arrangements; use only the gasket stack shown for the exact engine.
Is this a genuine Mercury kit?
No. It is a Sierra aftermarket replacement kit.
Important: Installing the wrong thermostat or gasket can cause overheating, overcooling or internal/external leaks. Confirm the exact engine serial-number parts diagram before service.
For the products under £150 the shipping is £6.99.
Free delivery for most items over £150.
