Mercury – 143° Thermostat | Cooling System Components | Precision Brass | 8M6010305

Overview

Every outboard has its own personality, but one thing they all share is the need for steady, predictable cooling—especially the hardworking two-strokes that many of us grew up trusting. The Quicksilver 8M6010305 143-degree thermostat steps into that role with the kind of quiet reliability that makes long runs feel effortless. When the water’s glassy and the engine settles into that familiar hum, you want to know the temperature inside the block is staying exactly where it should. That sense of confidence—of knowing your motor is protected and performing at its best—is what this thermostat delivers.

Key Features

• Maintains optimal 143°F operating temperature for select 90–170hp Evinrude, Johnson, and OMC two-stroke outboards
• Designed to meet or exceed all original factory specifications
• Direct replacement for OEM part number 435491 and compatible with additional cross-references
• Durable, marine-grade construction engineered for consistent thermal response
• Thermostat only; installation hardware and gaskets sold separately
• Simple replacement process following standard service-manual guidance

Engine & Boat Compatibility

• Fits select 90–170hp Evinrude, Johnson, and OMC V4 and V6 two-stroke outboards
• Suitable for classic fishing rigs, lightweight skiffs, mid-size offshore hulls, and restored two-stroke setups
• Ideal for engines used frequently in mixed-water environments such as brackish estuaries, inland rivers, and coastal runs
• Designed for motors that rely on precise temperature regulation for fuel economy, throttle response, and smooth idle

Performance & Practical Benefits

A thermostat may be small, but any boater who’s ever fought overheating or sluggish warm-ups knows just how big of a difference it makes. This Quicksilver thermostat delivers a steady, predictable temperature curve, helping your two-stroke get up to operating temp quickly and hold it without drifting. The result? Smoother starts on cool mornings, more responsive throttle transitions, and a cleaner-burning engine that doesn’t waste fuel trying to compensate for temperature swings.

Its tight, secure fit keeps water flowing exactly as the engineers intended. You’ll notice steadier running at idle, a more confident transition to plane, and better overall efficiency on long cruising stretches. And because the thermostat’s design matches OEM specs down to the smallest detail, there’s no guesswork—just a part that drops in and behaves exactly the way your engine expects.

Why Buy OEM

When it comes to cooling, precision matters. Aftermarket thermostats may run “close enough,” but close isn’t good enough when you’re miles from shore or trying to keep an older two-stroke running strong season after season. A genuine Quicksilver part gives you the correct temperature rating, correct dimensions, and the peace of mind that comes from using something engineered for long-term dependability. It’s the practical choice—not flashy, not complicated, just right.

FAQ

Q1: Which Mercury engines or systems does this thermostat fit?

This part is for select 90–170hp Evinrude, Johnson, and OMC V4 and V6 outboards—confirm fit using your engine’s model and serial number.

Q2: What makes this part different from aftermarket options?

It’s built to exact OEM specifications, ensuring proper thermal control and long-term reliability.

Q3: How do I know if it’s the correct replacement?

Match the part number (435491 replacement) and verify compatibility through your owner’s manual.

Q4: Can it be used in saltwater and freshwater?

Yes—this thermostat performs reliably in all marine environments.

Q5: Is installation straightforward?

Most boaters with basic mechanical comfort can handle the swap by following the service manual.