Mercury MerCruiser Thermostat 160°F | 8M0076464 | 496 MAG / 8.1L Big Block

Mercury MerCruiser Thermostat 160°F | Part 8M0076464

Part Number: 8M0076464
Brand: Mercury Marine / Quicksilver
Official Product Type: Engine Thermostat — 160°F (71°C)
Supersedes: 863358T
Engine Family: MerCruiser 8.1L Big-Block (GM-based)
Operation: Wax-element, spring-loaded valve

What It Does

The thermostat controls coolant flow through the engine to maintain the correct operating temperature. On the MerCruiser 8.1L big-block, the 160°F (71°C) calibration is the OEM specification — it keeps the engine in the temperature range required for proper combustion efficiency, fuel control, and long-term durability under marine load. The wax-element valve opens progressively as coolant temperature rises, allowing flow to the heat exchanger or raw-water circuit, and closes when the engine cools to maintain warm-up temperature.

A failed thermostat — stuck open or stuck closed — causes the engine to run too cold or overheat respectively, both of which accelerate wear and can trigger engine protection systems.

Supersedes / Cross Reference

• Current OEM Part: 8M0076464
• Supersedes: 863358T

If your parts diagram or service manual references 863358T, 8M0076464 is the current replacement.

Compatible Applications

Always verify fit using your engine serial number against the current Mercury parts diagram. The 8.1L big-block uses a different thermostat than small-block MerCruiser engines — temperature rating alone does not confirm fitment.

• MerCruiser 496 MAG (8.1L) — sterndrive
• MerCruiser 496 MAG HO (8.1L) — sterndrive
• MerCruiser 8.1L Inboard — V-drive and inline configurations
• MerCruiser 8.1L big-block GM-based marine engines — confirm by serial number

Serial Requirements

Thermostat fitment on MerCruiser big-block engines can vary by production range and cooling system configuration (closed cooling vs. raw-water). Always confirm your engine serial number against the Mercury parts diagram before ordering. If your engine has been converted to a closed-cooling system, verify the thermostat specification for that configuration separately.

System

Engine cooling system — thermostat housing / water distribution assembly. The thermostat sits in the thermostat housing on the front of the engine and controls flow between the engine block and the cooling circuit. On raw-water cooled 8.1L engines, it regulates flow to the raw-water heat exchanger. On closed-cooling systems, it controls flow to the heat exchanger core.

Kit Contents

• Thermostat only (1 piece)

The thermostat housing gasket is not included and must be replaced at the same time. See related parts below.

Related Parts

Typically replaced or inspected at the same time as the thermostat:

• Mercury MerCruiser Quicksilver Thermostat Housing Gasket 806871 — replace at every thermostat service
• Mercury MerCruiser Quicksilver Gasket-Thermostat 804878
• Mercury MerCruiser Quicksilver Thermostat Gasket 62386001
• Mercury MerCruiser Thermostat Kit 807252T5 — thermostat and gasket together
• Quicksilver Intake Manifold & Valley Gasket Set 8.1L 496 MAG – 27-881633T01 — if servicing the intake at the same time
• Mercury MerCruiser 160°F Thermostat 8M0109441 — alternate 160°F thermostat (confirm application)
• Sierra Marine Thermostat 18-43017 – MerCruiser 8.1L 496 MAG — aftermarket alternative

Service Interval

Replace the thermostat every 2–3 years or whenever the engine shows signs of temperature irregularity — overheating, running cold, or erratic temperature gauge readings. Always replace the thermostat housing gasket at the same time. On high-hour engines or those operating in salt water, annual inspection is recommended. Do not reuse a thermostat that has been stuck, corroded, or removed from a failed cooling system.

Installation Notes

• Install with the spring side facing into the engine block — incorrect orientation prevents the valve from opening properly
• Replace the thermostat housing gasket every time the housing is removed — do not reuse old gaskets
• Clean the housing mating surfaces thoroughly before installing the new gasket and thermostat
• Torque the housing bolts to the specification in the Mercury service manual — over-tightening cracks the housing
• Run the engine and verify the temperature gauge reaches and holds the correct operating range before returning to service

Technical FAQ

Can I use a 160°F thermostat from a small-block MerCruiser?
No. The 8.1L big-block uses a different thermostat diameter and housing design than small-block engines. The temperature rating is the same, but the physical fit is not interchangeable. Always use the part number specified for your engine.

My engine is running cold — is the thermostat stuck open?
A thermostat stuck in the open position is a common cause of an engine that won’t reach operating temperature. The engine will run rich, produce more carbon buildup, and wear faster. Replace the thermostat and housing gasket as a set.

My engine is overheating — is it the thermostat?
Overheating can be caused by a thermostat stuck closed, but also by a failed raw-water pump impeller, blocked heat exchanger, or low coolant. Diagnose the cooling system fully before assuming the thermostat is the cause. If the thermostat is suspect, it is inexpensive enough to replace as part of the diagnosis.

Does this fit closed-cooling (freshwater-cooled) 8.1L engines?
Verify against your engine serial number and the Mercury parts diagram for your specific cooling system configuration. Closed-cooling and raw-water cooled engines may use the same thermostat, but confirm before ordering.

Parts Desk Insights

Best use case: Scheduled thermostat replacement on MerCruiser 496 MAG and 8.1L big-block engines during annual service, or as part of a cooling system diagnosis on an engine with temperature irregularities. This is the correct OEM thermostat for this engine family — order it with the thermostat housing gasket to complete the job in one visit.

Common issues:

• Wrong thermostat ordered based on temperature rating alone — 160°F thermostats are used across many MerCruiser engine families, but they are not physically interchangeable. The 8.1L big-block requires this specific part. Ordering a small-block 160°F thermostat will not fit the housing.
• Gasket not replaced at the same time — the thermostat housing gasket should always be replaced when the thermostat is serviced. Reusing an old gasket is a common cause of coolant leaks after reassembly.
• Thermostat installed backwards — the spring must face into the engine. An incorrectly oriented thermostat will not open at the correct temperature and can cause overheating.
• Not verifying serial number before ordering — the 8.1L big-block was produced across multiple years and configurations. Confirm the serial number matches this part in the Mercury diagram before ordering, especially on repowered boats.