The Big Retainer Cover Nut, Mercury OEM Part Number 158401—often called the Big Nut, 2.5 SportMaster cover nut, or CLE retainer nut—is a heavy-duty upgrade for Mercury Racing outboards. This bearing carrier retainer secures the carrier inside the gear housing, maintaining gear alignment and bearing preload under extreme conditions.
It is a direct-fit for 2.0L, 2.4L, 2.5L, and 3.0L gearcases, including 200 cases, CLE gearcases, SportMaster lower units, and custom gearcases with low-water pickups. Unlike the standard ½-inch retainer, the 158401 Big Nut uses ¾-inch threads, providing stronger engagement, more durability, and longer service life in surfacing applications.
Original Applications
The 158401 retainer nut was standard on many Mercury high-performance outboards, especially those with CLE and SportMaster gearcases. Notable models include:
150 HP Pro Max / Super Mag (2.5L) – Paired with CLE lower units.
150 / 200 HP Pro Max / Super Mag (2.5L) – High-revving motors using Sporty carriers.
2.5L EFI Offshore models – Offshore engines with CLE gearcases.
200 HP Pro Max / Super Mag (2.5L) – Popular performance outboards with SportMasters.
200 HP XS (2.5L) and ROS versions – Racing engines with low-water pickups.
225 HP EFI Pro Max / Super Mag (2.5L) – High-torque motors requiring stronger carrier retention.
225X EFI Pro Max (2.5L) – A Mercury Racing flagship matched to SportMasters.
These 150–225 HP 2.5L motors relied on the Big Nut to keep the bearing carrier secure at high speeds.
Why the Big Nut Matters
The Nut for Sporty bearing carrier is critical in performance lower units. It locks the carrier firmly in the housing, preventing movement that can disrupt prop shaft alignment or gear mesh.
On 2.5 SportMaster lower units, CLE cases, and 200 cases with low-water pickups, loads are amplified by surfacing props, high X-dimensions, and racing conditions. The ¾-inch Big Nut was engineered to withstand these stresses, unlike the weaker ½-inch version.
The wider thread design provides greater engagement with the gear housing, resisting loosening and fatigue. It spreads load across more surface area, protecting both nut and housing.
By holding the carrier more securely, the CLE retainer nut keeps gears aligned and reduces wear, extending gearcase life. For saltwater or frequently serviced cases, the stronger thread also reduces galling and thread failure.
Installation Notes
Proper installation requires the Mercury carrier retainer wrench (tool 91-840393 or equivalent). Clean and chase threads, apply Mercury-approved anti-seize, and torque to spec from the service manual. Always pressure and vacuum test the gearcase before refilling with gear lube.
The Mercury 158401 Big Nut—also known as the 2.5 SportMaster cover nut, CLE retainer nut, and Nut for Sporty bearing carrier—remains the proven upgrade for Mercury Racing outboards.
From 150 HP Pro Max motors to the 225X EFI Pro Max, this ¾-inch carrier retainer nut delivers the strength, stability, and reliability needed for CLE gearcases, SportMaster lower units, and 200 cases with low-water pickups.
Contact Mike Hill with any tech questions at +1-714-697-1716 or mike@buckshotracing77.com