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 eng