Sierra Marine Thermostat 18-43017 Indmar S985009 / MerCruiser 8.1L 496 MAG Cooling System

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Alternate / Superseded Part Numbers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.