MerCruiser Gearcase Drain Screw Seal – 8M0187157 (Blue)

MerCruiser Gearcase Drain Screw Seal – 8M0187157

This small blue sealing washer is the OEM drain and fill screw seal for MerCruiser sterndrive gearcases and Mercury/Mariner outboard lower units. It sits under the drain and vent screws to create a leak-free seal that keeps gear lube in and water out. Mercury recommends replacing this seal every time the gear lube is changed — it’s a low-cost consumable that prevents costly leaks and contamination.

Compatible Applications

• MerCruiser Alpha One, Alpha One Gen II, and Bravo sterndrive gearcases
• Mercury and Mariner outboard lower units (most models, approximately 6–220 HP)
• Confirm fitment against your engine model and serial number — some Japanese-built models and certain Verado configurations use a different seal

Part Details

• OEM Part Number: 8M0187157
• Also Referenced As: 12-8M0187157, 12-19183-3
• Brand: Mercury / MerCruiser / Quicksilver
• Type: Gearcase drain and fill screw seal (blue compressible washer)
• System: Lower unit / gearcase
• Fits Screw Size: 3/8-16 drain and fill screws
• Material: Soft compressible sealing washer
• Condition: New OEM

Service Interval

Replace this seal every time gear lube is changed. Mercury Marine specifies seal replacement at each service interval to ensure a proper leak-free seal. Reusing the old seal is a common cause of gear lube leaks after service.

Technical FAQ

• Do I need one or two seals per service? Most gearcases have two screws — a drain screw and a vent/fill screw — so order two seals per service.
• What happens if I reuse the old seal? The compressible washer deforms on first use. Reusing it risks an incomplete seal, leading to gear lube leaks or water intrusion into the lower unit.
• Is this the same seal for all Mercury outboards? No — some Japanese-built Mercury models and certain Verado engines use a different drain screw configuration. Verify by model and serial number.
• Can I use a generic washer instead? Not recommended. The OEM blue seal is made from a specific compressible material engineered to seal properly at the correct torque spec. Generic substitutes may not compress correctly.

Related Parts

• MerCruiser High Performance Gear Lube – 8M0222063
• Mercury MerCruiser Gear Lube Pump Kit – 8M0072135
• Mercury Lower Unit Seal Kit – 26-33144A2
• Mercury MerCruiser Blue Seal 8M0187157 (alternate listing)

Parts Desk Insights

This is one of the most commonly forgotten parts during a gear lube service — technicians often focus on the lube itself and overlook the drain seal. Always order at least two per service (one for the drain screw, one for the vent/fill screw). The most common mistake is reusing the old seal: the blue washer is a one-time-use compressible gasket that deforms on installation, and reusing it is a leading cause of post-service gear lube leaks. If you’re servicing multiple drives or outboards, buy a handful at a time — at this price point, there’s no reason to risk a leak. Confirm your engine model and serial number if you’re unsure whether this seal fits your specific configuration.