Mercury – Thermostat, 160°F | Cooling System | 863358T / 8M0076464

Overview

Big-block MerCruiser engines like the 496 Magnum, 496 HO, and the 8.1L series run hot, work hard, and rely heavily on a good thermostat to keep temperatures steady. When the thermostat starts sticking—or worse, won’t open at all—you’ll see it right away on the gauge. This 160°F thermostat is the correct replacement for those engines and brings the cooling system back to where it should be. It’s a simple part, but on these big motors, it makes a noticeable difference in how they warm up and hold temperature.

Key Features

 • 160°F Marine Thermostat – Correct heat range for 496 Magnum, 496 HO, and 8.1L engines.
 • Straightforward Replacement – Swaps out easily with basic tools.
 • Stable Temperature Control – Helps the engine warm evenly and avoid hot spots.
 • OEM-Spec Build – Designed to handle marine heat cycles and constant vibration.
 • Reliable Sealing Surface – Fits the housing the way Mercury intended.

Engine & Boat Compatibility

 • MerCruiser 496 Magnum
 • MerCruiser 496 HO
 • MerCruiser 8.1L
 • MerCruiser 8.1L HO
 • Sterndrive and inboard cooling systems

Performance & Practical Benefits

A fresh thermostat helps these big engines settle into their normal temperature quickly and stay there—no wandering up and down the gauge, no sluggish warm-up, no sudden heat spikes. It also helps fuel mixture and timing behave correctly, since these systems depend on stable temperature. On older engines with hours on them, this little part often clears up rough idle or odd running quirks tied to poor temperature control.

Why Buy OEM

OEM thermostats are built for marine duty: heavy heat cycles, constant load, and the vibration that shakes cheaper parts apart. They open at the right temp, seal correctly, and don’t stick halfway after a season or two. With big-block MerCruisers, the thermostat isn’t where you want to gamble on an off-brand part.

FAQ

Q1: How do I know my thermostat is failing?
A: Temperature creeping higher than usual, slow warm-up, inconsistent temp readings, or overheating at idle.

Q2: Can I replace this at home?
A: Yes—most boaters can handle it with basic hand tools.

Q3: Is 160°F the correct temperature for these engines?
A: Yes—that’s the factory spec for the listed MerCruiser models.

Q4: Should I replace the gasket too?
A: It’s a good idea—if it’s worn or brittle, replace it while you’re in there.

Q5: Will a bad thermostat hurt performance?
A: Absolutely. Wrong temperatures can cause rough running, poor fuel economy, and overheating.

• Supersessions
  8M0076464, 8M0076464