Sierra 18-0378 Powerhead Mounting Gasket for Mercury/Mariner V6 Outboard

The Sierra 18-0378 is a Powerhead Base Plate Mounting Gasket engineered as a direct aftermarket replacement for Mercury and Mariner V6 2-stroke outboard engines. This gasket seats between the powerhead and midsection, providing a critical seal that prevents cooling water from migrating into the exhaust system and robbing the engine of performance.

Manufactured from AFM 30 (Aluminum Faced Material) — the same Victor Reinz-grade compound used in high-output applications — this gasket delivers superior resistance to warping under the high RPMs and repeated thermal cycling demands of marine V6 powerplants. It is precision-cut to match OEM bolt hole and port patterns for a leak-free fit.

Key Features:

• Material: AFM 30 (Aluminum Faced Material) — premium Victor Reinz compound
• Type: Powerhead Base Plate / Exhaust Plate Gasket
• Sold individually (1 per package); V6 Race Cowl with plate applications require 2
• Meets or exceeds OEM specifications
• Direct replacement for Mercury OEM part numbers: 27-8231422, 27-8231423, 27-78069, 27-823142A94

Cross-Reference Part Numbers:

• Sierra: 18-0378
• Mercury OEM: 27-8231422, 27-8231423, 27-78069, 823142, 8231421, 8231422, 8231423
• Mallory: 9-60045
• GLM / Other: 31530, 37620, 510-38

Compatible with Mercury and Mariner 2.0L, 2.4L, and 2.5L V6 2-stroke outboards from 1980–2005, including Black Max, XR2, XR4, XR6, Magnum II, Magnum III EFI and Carbureted models, Pro Max, and high-performance 260 ROS, 280, and 300 Drag Race engines.