If you have ever opened a Mercury or MerCruiser parts diagram, you will have noticed that an enormous number of part numbers begin 25-. That prefix is Mercury Marine's category code for seals, o-rings and gaskets. It tells you what kind of part you are looking at, not where it goes.
Where sterndrive o-rings fail
Marine seals live a harder life than almost anything in our catalogue. They see water, temperature swings, long periods of no use, and in salt water, constant corrosion at the sealing faces.
- Gimbal housing and bell housing. Water intrusion here is the classic sterndrive failure, and it usually starts as a seal that has taken a set over a winter layup.
- Drive shaft and u-joint bellows area. Rings that have hardened will pass water into the transom assembly.
- Water pump and cooling circuit. Seals that have gone brittle cause overheating long before they cause a visible leak.
- Drain and fill plugs. Cheap, routinely reused, and a very common source of slow gear oil weep. Replace them every service — a plug seal costs a fraction of a lower unit.
Milky gear oil means act now
If your lower unit oil comes out looking like coffee with cream, water is getting in past a seal. That emulsion does not lubricate. Continuing to run it destroys the gearset, which is an order of magnitude more expensive than the seal that let the water in.
Common MerCruiser o-rings we stock
We carry a range of MerCruiser, Mercury, Mariner and Force seals. If you have a 25- number that is not listed, send it to us — we can usually cross-reference it.
Material matters in salt water
Buna-N nitrile is the correct default for marine seals in contact with water and gear oil. Where a ring sees fuel or aggressive chemistry, FKM is the better choice. Our o-ring material guide covers the differences.
Before you reassemble
Seat every o-ring with a thin film of food-grade silicone lubricant. Dry rings pinch and take a set, which is how a new seal weeps within a season. Aqua Flex Food Grade Silicone Lubricant is what ships in our kits.
O-rings only. Never lubricate the polished carbon or ceramic faces of a mechanical shaft seal — they must run dry against each other or the seal slips and leaks from new.
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.