Mercury Marine absorbed both Mariner and Force, and the parts catalogues merged along with the companies. A seal listed under one brand very often fits the others.
The 25- prefix
Mercury's category code for seals, o-rings and gaskets is 25-. It tells you the kind of part, not its location. See our MerCruiser 25- series guide for the sterndrive side of the same numbering.
Where outboards leak
Lower unit
Check the gear oil at every service. Milky or coffee-coloured means water is getting past a seal, and that emulsion does not lubricate. Continuing to run it costs you the gearset.
Water pump housing
Impeller replacement is an annual job on most outboards. Replace the housing o-rings at the same time rather than reusing them — you are already there, and the parts are inexpensive.
Drain and fill plug seals
Replace, do not reuse. They are among the cheapest parts on the engine and a very common slow weep.
Thermostat and cover seals
Hardened cover seals cause overheat alarms that get misdiagnosed as thermostat or impeller faults.
Fuel system
Ethanol-blended fuel is hard on older elastomers. Engines that predate E10 and still carry original fuel system rings should have them treated as consumables.
Winter is what kills them
Outboard seals mostly fail sitting still — compressed, cold, for months — not underway. See our spring commissioning checklist for what to inspect before launch.
For OMC-family engines see Johnson, Evinrude, OMC and Volvo Penta.
Before you reassemble
Seat every o-ring with a thin film of food-grade silicone. Dry rings pinch and take a set. Aqua Flex Food Grade Silicone Lubricant is what ships in our kits.
O-rings only. Never lubricate the polished faces of a mechanical shaft seal.
Beaver Island Parts Company is not affiliated with, endorsed by, or sponsored by any equipment manufacturer named on this page. All brand names, model numbers and part numbers are the property of their respective owners and are used solely to identify the equipment our replacement parts fit.