Description

The Indmar Thermostat 986115D is a genuine OEM 160°F drilled thermostat designed for GM Gen IV LY6 and L96 marine engines equipped with closed cooling systems. This thermostat features a precision 5/32” bypass (drilled) hole, allowing controlled coolant flow during warm-up to reduce thermal shock and improve temperature stability.

Engineered specifically for Indmar applications, this thermostat ensures consistent operating temperature, optimized combustion efficiency, and proper coolant circulation through the heat exchanger system.

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Key Features

• Genuine Indmar OEM thermostat
• 160°F operating temperature
• Integrated drilled bypass (5/32”) for improved coolant flow
• Designed for closed cooling (freshwater cooled) systems
• Maintains stable engine temperature under load
• Direct replacement — no modification required

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Engine Compatibility

Fits Indmar GM Gen IV closed cooled engines:

• LY6 6.0L V8
• L96 6.0L V8

Applications include:

• MasterCraft, Malibu, Supra, Moomba, and other Indmar-powered inboards
• Closed cooling / heat exchanger equipped engines

Specifically for closed cooling systems
Not for raw water cooled thermostat housings

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Serial Number Fitment Notice

Fitment varies by engine configuration and model year. Always confirm using your engine serial number or engine model (LY6/L96) before ordering.

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When This Part Needs Replacement

• Engine running too hot or overheating
• Engine not reaching or holding temperature
• Temperature fluctuations under load
• Thermostat sticking or corrosion present
• Cooling system service interval

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Installation Notes

• Install with spring side toward engine
• Ensure drilled bypass hole is oriented correctly (typically upward)
• Replace gasket/seal during installation (sold separately)
• Inspect heat exchanger and coolant passages
• Verify coolant condition before reassembly

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Alternate / Superseded / Cross-Reference Part Numbers

• Indmar: 986115D
• Related (non-drilled variant): 986115
• Comparable applications: GM Gen IV 6.0L marine thermostats

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Parts Desk Insight

The drilled bypass is the key difference on this thermostat. It allows a small amount of coolant to circulate before the thermostat fully opens, which helps prevent hot spots and improves temperature stability — especially important on closed cooling Indmar setups. Installing a non-drilled thermostat in these engines can lead to erratic temperature behavior.

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Shipping

Shipping: Ships same or next day for fast Canada wide shipping.