Keep your big-block MerCruiser running cool and clean with Mercury’s Thermostat Seal 25-8071341. This low-profile ring nests between the thermostat cup and housing on closed-cooling V-6 and V-8 MCM/MIE engines, locking in pressure so coolant circulates exactly the way the factory intended.

Why this seal is worth ten minutes on the workbench

• OEM geometry—matches the 3.25-inch groove on popular 4.3L, 5.0L, 5.7L, and 8.2L blocks; no trimming or RTV.

• Fuel- and glycol-resistant EPDM shrugs off 250 °F heat and the corrosion inhibitors in Dex-Cool or Prestone-style mixes.

• Squared sealing lip distributes torque evenly, stopping that slow weep that leaves pink crust around the housing.

• One-piece drop-in eliminates messy paper gaskets and keeps future services simple.

Swap it during a spring impeller change or whenever you inspect the thermostat. The payoff is immediate: tighter temperature control, zero coolant loss, and the confidence to lean on the throttle without eyeing the gauge. Small part, big insurance for the heart of your Mercury powerplant.