Mercury – Quicksilver 8M0204651 Thermostat Housing O-Ring | MerCruiser V6 & V8 Series | Rubber Seal | 8M0204651

Overview

There’s a certain comfort that comes with knowing your cooling system is sealed up the way it should be. You feel it on those long runs when the engine hums at a steady temperature, never wavering, never giving you a reason to second-guess what’s happening under the cover. This Mercury Marine part plays a quiet but essential role in that peace of mind. It’s the kind of simple component you don’t think about often until the day you really need it to hold tight and do its job without fail.

Key Features.

• Thermostat housing O-ring designed to create a tight, leak-free seal
• Made from durable marine-grade rubber for long-lasting performance
• Intended for housings with a flat cover plate
• Built to Mercury Marine part number 8M0204651 specifications
• Replaces previous part number 30224
• Resistant to heat, pressure, and constant engine vibration

Engine & Boat Compatibility

• Fits MerCruiser marinized V6 and V8 engines based on General Motors platforms
• Compatible with both sterndrive and inboard cooling systems using this style of thermostat housing
• Common on cruisers, runabouts, bowriders, and larger recreational boats.
• Suitable for many freshwater and saltwater sterndrive rigs

Performance & Practical Benefits

A well-sealed thermostat housing is something you feel through the entire boat — in the steady temperature gauge, the smooth engine note, and the confidence you have during long days away from the dock. This O-ring creates a tight, secure seal that holds steady against heat cycles, pressure changes, and constant vibration. Once it’s seated properly, it keeps coolant exactly where it needs to be, preventing leaks that could lead to overheating or more costly repairs.

The rubber compound is flexible enough to install easily, yet firm enough to stay in place once compressed between the housing and cover plate. You’ll notice how clean the fit feels — no bunching, no awkward stretching, just a smooth installation that locks in place like it was made for your engine, because it was. Over time, the durable construction helps reduce maintenance headaches, extending the life of your cooling components and giving you more freedom to focus on the water instead of the wrench.

Why Buy OEM

Cooling system seals might seem like minor parts, but anyone who’s dealt with a leak knows how quickly things can go wrong. Choosing a genuine Mercury Marine part means you’re using a seal made to the exact dimensions and materials your engine requires. Aftermarket O-rings often vary in thickness or rubber quality, leading to loose fits, premature failure, or small leaks that grow into big problems.

Using the correct part number ensures the thermostat housing closes evenly and securely, maintaining proper engine temperature and protecting your investment. It’s practical advice seasoned boaters stick to: go with OEM for seals, and your engine will reward you with reliability season after season.

FAQ

Q1: Which engines does the Thermostat Housing O-Ring 8M0204651 fit?

It fits MerCruiser marinized V6 and V8 engines built on General Motors platforms.

Q2: What makes this O-ring better than aftermarket versions?

It matches Mercury’s exact sizing, material quality, and compression specs, ensuring a tighter, longer-lasting seal.

Q3: How can I confirm it’s the right replacement for my setup?

If your thermostat housing uses a flat cover plate and your manual calls for part number 8M0204651 or 30224, this is the correct part.

Q4: Can it be used in both saltwater and freshwater boats?

Yes — the rubber compound is designed for marine environments and holds up well in both conditions.

Q5: Is installation straightforward?

Very. Clean the housing surfaces, seat the O-ring properly, reinstall the cover, and the seal forms cleanly with even compression.

• Supersessions
  25-30224, 25-30224Q, 25-8M0204651, 25-8M0204651