Sierra Marine Thermostat 18-43017 – Indmar S985009 / MerCruiser 8.1L 496 MAG Cooling System
Free returns on all eligible orders
You have 30 days to request a return. All sale items are final sale.
The Sierra 18-43017 Thermostat is a precision 160° temperature control thermostat designed for high-horsepower GM-based marine engines, including Indmar, MerCruiser, and other big-block applications.
This thermostat regulates coolant flow to maintain optimal engine operating temperature, ensuring proper combustion efficiency, engine protection, and consistent performance under load.
Engineered to meet or exceed OEM specifications, this unit is a direct replacement for Indmar S985009 and multiple MerCruiser thermostat applications used in 8.1L platforms.
Key Features
- 160° thermostat for optimal marine engine operating temperature
- Designed for raw water cooled GM marine engines
- Improves engine efficiency and prevents overheating
- Direct OEM replacement fit and performance
- Corrosion-resistant marine-grade construction
- Sierra limited lifetime warranty
Engine Compatibility
MerCruiser (Big Block GM Engines)
- 8.1L (496 MAG)
- 496 MAG HO
- 8.1L MIE / Horizon
Indmar Marine Engines
- GM-based 8.1L engines
- Ford & GM raw water cooled applications (select models)
➡️ Specifically designed for high-output big block engines with raw water cooling systems
Serial Number Fitment Notice
Thermostats vary by:
- Temperature rating (160° vs others)
- Housing design
- Cooling system type
⚠️ Always confirm fitment using engine serial number or OEM part number before ordering.
When This Part Needs Replacement
Replace the thermostat if you experience:
- Engine overheating or running cold
- Inconsistent temperature readings
- Poor fuel efficiency
- Slow warm-up or stuck thermostat
- Corrosion or scaling inside housing
A failed thermostat can cause the engine to run outside optimal temperature and reduce performance.
Installation Notes
- Replace gasket or seal during installation
- Clean mating surfaces thoroughly
- Install in correct orientation
- Confirm 160° rating matches OEM spec
- Test engine temp after installation
Alternate / Superseded Part Numbers
OEM Cross References
- Indmar: S985009
-
Mercury / MerCruiser:
- 863358T
- 863358001
- 8M0074922
- 8M0076464
- 8M0192249
Aftermarket Cross References
- Mallory: 9-43017
Shipping
Shipping: Ships same or next day for fast Canada wide shipping. ⚓
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 3–4 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
Returns & Satisfaction Guarantee
We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
We are serious about product integrity — and equally serious about taking care of our customers. If there is ever a product issue, contact us immediately so we can make it right.
For return authorization, please contact our support team before shipping anything back.