In marine propulsion systems, especially on Mercury outboards between 75 and 300 horsepower, proper propeller installation is critical to drivetrain longevity and performance. One often-overlooked yet essential component in this assembly is the Z-Tab Washer with a .770-inch inner diameter, known by OEM part number 8M0042130, now superseded by 8M0204724.
This washer functions as part of the propeller retention system. It is installed beneath the propeller nut to prevent the nut from backing off under load. The tabbed design ensures mechanical engagement with the trim tab or adapter, locking the washer in place and transmitting torque effectively while resisting rotational movement. The proper seating and retention of this washer directly affect the axial positioning of the propeller hub and drive sleeve.
Manufactured from zinc-plated alloy steel, the washer offers corrosion resistance in both saltwater and freshwater environments. The .770-inch ID is a critical fit specification that matches the propeller shaft dimensions used in Mercury’s larger outboard platforms and some sterndrive units.
(It is important not to confuse this washer with smaller-diameter variants—such as the .642-inch ID version used on lower-horsepower outboards—as incorrect fitment can lead to improper torque retention, misalignment, and accelerated wear on the propeller shaft or splines.)
This part is specified for use on:
Mercury 75–300 hp outboards (2-stroke and 4-stroke)
Mercury 40–60 hp BigFoot FourStroke models
MerCruiser Alpha and Bravo One sterndrives
When servicing or replacing a propeller on these systems, the Z-Tab Washer should always be inspected and replaced if worn, deformed, or corroded. Proper reinstallation includes ensuring the tab aligns with the mating component (typically the anode or trim tab assembly) and that the prop nut is torqued to factory specifications using a calibrated torque wrench.
Failure to use the correct washer can result in axial play, propeller chatter, or loosening of the nut during operation. Over time, this can damage the shaft taper, shear the key, or deform the hub insert—resulting in costly repairs.
In summary, the Mercury Z-Tab Washer (.770 ID) is a mission-critical component for proper propeller retention in high-output marine engines. For accurate and safe installation, always confirm the washer's dimensions and OEM part number—8M0042130 or 8M0204724—before replacement.
Contact Mike Hill at mike@buckshotracing77.com or +1-714-697-1716