Sierra 18-0578

Upgrade Your Outboard with Reliable Crankshaft Protection

Keeping your Mercury outboard running smooth means paying attention to the details—like the crankshaft oil seal that stands guard against leaks and contamination. This precision-engineered seal delivers the durability boaters demand for those long days on the water, ensuring your engine stays lubricated and protected from the harsh marine environment.

Boaters know the frustration of oil spots under the motor after a trip. This seal steps in to prevent that, using high-quality materials that resist heat, pressure, and saltwater corrosion better than stock options in many cases. Whether you're prepping for fishing season or routine maintenance, it's a smart swap that extends engine life.

Perfect Fit for Mercury Powerplants

Designed as a direct replacement for Mercury part 26-67388, this seal matches OEM specs for seamless integration. It fits a range of 90-125 HP inline 4-cylinder engines, including popular models from the 1980s through early 2000s. Always cross-check your engine serial number to confirm compatibility, but it's a go-to for carbureted and EFI setups alike.

• Supports 3.0L and 3.7L Mercury/Mariner outboards
• Ideal for both freshwater and saltwater applications
• Engineered for upper crankshaft position to seal in critical lubricants

Pro tip: If you're pulling the powerhead for other work, this is the perfect time to inspect and replace surrounding seals for a complete refresh.

Key Features That Outperform

What sets this apart? It's not just a seal—it's built with advanced lip design for superior oil retention under high RPMs. The material blend handles temperatures up to 300°F without hardening or cracking, a common failure point in cheaper alternatives.

Step-by-Step Installation Guide

Swapping this seal is straightforward with basic tools, but patience pays off. Here's how pros do it:

1. Prep the workspace: Drain oil, remove cowling, and secure the outboard upright.
2. Access the crankshaft: Remove flywheel or powerhead as needed—use a seal puller to extract the old one without scoring the shaft.
3. Clean thoroughly: Wipe the shaft and housing with brake cleaner; any debris spells trouble.
4. Install new seal: Lightly lube the outer edge with marine grease, then tap into place with a seal driver or socket that matches the OD. Face the lip correctly—downward for upper seals.
5. Reassemble and test: Torque to spec, refill oil, run the motor on muffs, and check for drips after 30 minutes.

Common pitfall: Overtightening fasteners can distort the seal. If you're new to this, watch for shaft wear—hone lightly if scored to avoid premature failure.

Why Boaters Trust Sierra Engine Parts

Sierra has been the aftermarket king for marine engines for decades, supplying parts that mirror Mercury quality at a fraction of dealer costs. Their seals undergo rigorous testing for seal integrity, material fatigue, and environmental exposure—real-world abuse that weekend warriors and charter captains put their motors through.

Owners report 2-3x longer service intervals compared to generics, with fewer comebacks during peak season. Pair it with fresh gear lube and filters for a bulletproof lower unit.

Spotting Seal Failure Early

Don't wait for a puddle. Watch for blue smoke on startup, low oil levels without visible external leaks, or milky gear oil indicating water intrusion. Regular compression tests and oil analysis catch issues before they strand you miles offshore.

Maintenance hack: Change lower unit oil twice a season in saltwater, and inspect seals during annual service. Your wallet (and tow bill) will thank you.

Frequently Asked Questions