Mercury O-Ring – 25-863006

This tiny ring—about the size of a dime at 0.50 in. across—does the heavy lifting of sealing critical fuel- and coolant-passages on dozens of MerCruiser MPI and big-block inboards, from 4.3-liter Alpha/Bravo packages up to 8.2-liter MAG HO models. Molded from fuel-grade nitrile and trimmed to a precise 0.13-in. profile, it compresses evenly in the groove, locking out air bubbles that cause lean pops and blocking the seepage that invites corrosion. At just 0.01 lb, it’s easy to overlook—until a misty drip shows up on the bilge pad.

Why it belongs in every spare-parts pouch:

• Exact OEM geometry mates perfectly with Mercury manifolds, pump housings, and sensor bosses—no trimming or RTV.
• Fuel- and glycol-resistant compound stands up to ethanol blends, Dex-Cool, and salty bilge water longer than bargain gaskets.
• Color-coded black finish makes quick work of visual inspections during annual service.
• Fits an extensive list of 5.0L, 5.7L, 6.2L, 8.1L, and 8.2L engines (see serial ranges), reducing guesswork at the parts counter.

Swap it whenever you crack a fitting or notice weeping around a sender. A pocket-sized seal now saves hours of chasing coolant smells or fuel stains when you’d rather be chasing the next set of rollers.